xfs: scrub directory parent pointers

xfs: scrub directory parent pointers

Scrub parent pointers, sort of.  For directories, we can ride the
'..' entry up to the parent to confirm that there's at most one
dentry that points back to this directory.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>