xfs: check that dir block entries don't off the end of the buffer

When we're checking the entries in a directory buffer, make sure that
the entry length doesn't push us off the end of the buffer.  Found via
xfs/388 writing ones to the length fields.

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