xfs: convert a few more directory asserts to corruption

Yet another round of playing whack-a-mole with directory code that
asserts on corrupt on-disk metadata when it really should be returning
-EFSCORRUPTED instead of ASSERTing.  Found by a xfs/391 crash while
lastbit fuzzing of ltail.bestcount.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>