xfs/278: find sfdir inode field prefix

Use the xfs set/get metadata field helpers to detect the correct sfdir
field name prefix on v4-v5 filesystems.  This enables us to test inode
link count corrections on a (deliberately) disconnected directory.

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