Extract the dst lookup logic from ipip6_tunnel_xmit() into new helper
ipip6_tunnel_dst_find() to reduce code duplication and enhance readability.
No functional change intended.

On a x86_64, with allmodconfig object size is also reduced:

./scripts/bloat-o-meter net/ipv6/sit.o net/ipv6/sit-new.o
add/remove: 5/3 grow/shrink: 3/4 up/down: 1841/-2275 (-434)
Function                                     old     new   delta
ipip6_tunnel_dst_find                          -    1697   +1697
__pfx_ipip6_tunnel_dst_find                    -      64     +64
__UNIQUE_ID_modinfo2094                        -      43     +43
ipip6_tunnel_xmit.isra.cold                   79      88      +9
__UNIQUE_ID_modinfo2096                       12      20      +8
__UNIQUE_ID___addressable_init_module2092       -       8      +8
__UNIQUE_ID___addressable_cleanup_module2093       -       8      +8
__func__                                      55      59      +4
__UNIQUE_ID_modinfo2097                       20      18      -2
__UNIQUE_ID___addressable_init_module2093       8       -      -8
__UNIQUE_ID___addressable_cleanup_module2094       8       -      -8
__UNIQUE_ID_modinfo2098                       18       -     -18
__UNIQUE_ID_modinfo2095                       43      12     -31
descriptor                                   112      56     -56
ipip6_tunnel_xmit.isra                      9910    7758   -2152
Total: Before=72537, After=72103, chg -0.60%

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
---
v2: add newlines before return in ipip6_tunnel_dst_find()
    add bloat-o-meter info in commit log
---
 net/ipv6/sit.c | 95 ++++++++++++++++++++++++--------------------------
 1 file changed, 45 insertions(+), 50 deletions(-)

diff --git a/net/ipv6/sit.c b/net/ipv6/sit.c
index 12496ba1b7d4..60bd7f01fa09 100644
--- a/net/ipv6/sit.c
+++ b/net/ipv6/sit.c
@@ -848,6 +848,49 @@ static inline __be32 try_6rd(struct ip_tunnel *tunnel,
 	return dst;
 }
 
+static bool ipip6_tunnel_dst_find(struct sk_buff *skb, __be32 *dst,
+				  bool is_isatap)
+{
+	const struct ipv6hdr *iph6 = ipv6_hdr(skb);
+	struct neighbour *neigh = NULL;
+	const struct in6_addr *addr6;
+	bool found = false;
+	int addr_type;
+
+	if (skb_dst(skb))
+		neigh = dst_neigh_lookup(skb_dst(skb), &iph6->daddr);
+
+	if (!neigh) {
+		net_dbg_ratelimited("nexthop == NULL\n");
+		return false;
+	}
+
+	addr6 = (const struct in6_addr *)&neigh->primary_key;
+	addr_type = ipv6_addr_type(addr6);
+
+	if (is_isatap) {
+		if ((addr_type & IPV6_ADDR_UNICAST) &&
+		    ipv6_addr_is_isatap(addr6)) {
+			*dst = addr6->s6_addr32[3];
+			found = true;
+		}
+	} else {
+		if (addr_type == IPV6_ADDR_ANY) {
+			addr6 = &ipv6_hdr(skb)->daddr;
+			addr_type = ipv6_addr_type(addr6);
+		}
+
+		if ((addr_type & IPV6_ADDR_COMPATv4) != 0) {
+			*dst = addr6->s6_addr32[3];
+			found = true;
+		}
+	}
+
+	neigh_release(neigh);
+
+	return found;
+}
+
 /*
  *	This function assumes it is being called from dev_queue_xmit()
  *	and that skb is filled properly by that function.
@@ -867,8 +910,6 @@ static netdev_tx_t ipip6_tunnel_xmit(struct sk_buff *skb,
 	__be32 dst = tiph->daddr;
 	struct flowi4 fl4;
 	int    mtu;
-	const struct in6_addr *addr6;
-	int addr_type;
 	u8 ttl;
 	u8 protocol = IPPROTO_IPV6;
 	int t_hlen = tunnel->hlen + sizeof(struct iphdr);
@@ -878,28 +919,7 @@ static netdev_tx_t ipip6_tunnel_xmit(struct sk_buff *skb,
 
 	/* ISATAP (RFC4214) - must come before 6to4 */
 	if (dev->priv_flags & IFF_ISATAP) {
-		struct neighbour *neigh = NULL;
-		bool do_tx_error = false;
-
-		if (skb_dst(skb))
-			neigh = dst_neigh_lookup(skb_dst(skb), &iph6->daddr);
-
-		if (!neigh) {
-			net_dbg_ratelimited("nexthop == NULL\n");
-			goto tx_error;
-		}
-
-		addr6 = (const struct in6_addr *)&neigh->primary_key;
-		addr_type = ipv6_addr_type(addr6);
-
-		if ((addr_type & IPV6_ADDR_UNICAST) &&
-		     ipv6_addr_is_isatap(addr6))
-			dst = addr6->s6_addr32[3];
-		else
-			do_tx_error = true;
-
-		neigh_release(neigh);
-		if (do_tx_error)
+		if (!ipip6_tunnel_dst_find(skb, &dst, true))
 			goto tx_error;
 	}
 
@@ -907,32 +927,7 @@ static netdev_tx_t ipip6_tunnel_xmit(struct sk_buff *skb,
 		dst = try_6rd(tunnel, &iph6->daddr);
 
 	if (!dst) {
-		struct neighbour *neigh = NULL;
-		bool do_tx_error = false;
-
-		if (skb_dst(skb))
-			neigh = dst_neigh_lookup(skb_dst(skb), &iph6->daddr);
-
-		if (!neigh) {
-			net_dbg_ratelimited("nexthop == NULL\n");
-			goto tx_error;
-		}
-
-		addr6 = (const struct in6_addr *)&neigh->primary_key;
-		addr_type = ipv6_addr_type(addr6);
-
-		if (addr_type == IPV6_ADDR_ANY) {
-			addr6 = &ipv6_hdr(skb)->daddr;
-			addr_type = ipv6_addr_type(addr6);
-		}
-
-		if ((addr_type & IPV6_ADDR_COMPATv4) != 0)
-			dst = addr6->s6_addr32[3];
-		else
-			do_tx_error = true;
-
-		neigh_release(neigh);
-		if (do_tx_error)
+		if (!ipip6_tunnel_dst_find(skb, &dst, false))
 			goto tx_error;
 	}
 
-- 
2.34.1