Two functions in fs/jfs/jfs_imap.c compute agno from on-disk data
without checking agno < MAXAG before indexing im_agctl[MAXAG] and
im_aglock[MAXAG]. The sibling function diNewExt() already has the
correct guard — this patch adds the same check to diIFree() and
diAllocBit() for consistency.

Signed-off-by: Nikita Safronov <npetr9395@gmail.com>