xfs: do not use logged xattr updates on V4 filesystems

V4 superblocks do not contain the log_incompat feature bit, which means
that we cannot protect xattr log items against kernels that are too old
to know how to recover them.  Turn off the log items for such
filesystems and adjust the "delayed" name to reflect what it's really
controlling.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/libxfs/xfs_attr.c

@@ -982,7 +982,7 @@ xfs_attr_set(
 	int			error, local;
 	int			rmt_blks = 0;
 	unsigned int		total;
-	int			delayed = xfs_has_larp(mp);
+	bool			use_logging = xfs_has_larp(mp);
 
 	if (xfs_is_shutdown(dp->i_mount))
 		return -EIO;
@@ -1027,7 +1027,7 @@ xfs_attr_set(
 		rmt_blks = xfs_attr3_rmt_blocks(mp, XFS_XATTR_SIZE_MAX);
 	}
 
-	if (delayed) {
+	if (use_logging) {
 		error = xfs_attr_use_log_assist(mp);
 		if (error)
 			return error;
@@ -1101,7 +1101,7 @@ xfs_attr_set(
 out_unlock:
 	xfs_iunlock(dp, XFS_ILOCK_EXCL);
 drop_incompat:
-	if (delayed)
+	if (use_logging)
 		xlog_drop_incompat_feat(mp->m_log);
 	return error;
 

fs/xfs/libxfs/xfs_attr.h

@@ -31,7 +31,8 @@ struct xfs_attr_list_context;
 static inline bool xfs_has_larp(struct xfs_mount *mp)
 {
 #ifdef DEBUG
-	return xfs_globals.larp;
+	/* Logged xattrs require a V5 super for log_incompat */
+	return xfs_has_crc(mp) && xfs_globals.larp;
 #else
 	return false;
 #endif