xfs: require a relatively recent V5 filesystem for LARP mode

While reviewing the FIEXCHANGE code in XFS, I realized that the function
that enables logged xattrs doesn't actually check that the superblock
has a LOG_INCOMPAT feature bit field.  Add a check to refuse the
operation if we don't have a V5 filesystem...

...but on second though, let's require either reflink or rmap so that we
only have to deal with LARP mode on relatively /modern/ kernel.  4.14 is
about as far back as I feel like going.

Seeing as LARP is a debugging-only option anyway, this isn't likely to
affect any real users.

Fixes: d9c61ccb ("xfs: move xfs_attr_use_log_assist out of xfs_log.c")
Really-Fixes: f3f36c89 ("xfs: Add xfs_attr_set_deferred and xfs_attr_remove_deferred")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Bill O'Donnell <bodonnel@redhat.com>

fs/xfs/xfs_xattr.c

@@ -46,6 +46,17 @@ xfs_attr_grab_log_assist(
 	if (xfs_sb_version_haslogxattrs(&mp->m_sb))
 		return 0;
 
+	/*
+	 * Check if the filesystem featureset is new enough to set this log
+	 * incompat feature bit.  Strictly speaking, the minimum requirement is
+	 * a V5 filesystem for the superblock field, but we'll require rmap
+	 * or reflink to avoid having to deal with really old kernels.
+	 */
+	if (!xfs_has_reflink(mp) && !xfs_has_rmapbt(mp)) {
+		error = -EOPNOTSUPP;
+		goto drop_incompat;
+	}
+
 	/* Enable log-assisted xattrs. */
 	error = xfs_add_incompat_log_feature(mp,
 			XFS_SB_FEAT_INCOMPAT_LOG_XATTRS);