xfs: teach scrub to flag non-extents format cow forks

CoW forks only exist in memory, which means that they can only ever have
an incore extent tree.  Hence they must always be FMT_EXTENTS, so check
this when we're scrubbing them.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

fs/xfs/scrub/bmap.c

@@ -748,6 +748,8 @@ xchk_bmap(
 	case XFS_DINODE_FMT_DEV:
 	case XFS_DINODE_FMT_LOCAL:
 		/* No mappings to check. */
+		if (whichfork == XFS_COW_FORK)
+			xchk_fblock_set_corrupt(sc, whichfork, 0);
 		goto out;
 	case XFS_DINODE_FMT_EXTENTS:
 		break;