tune2fs: warn if extents are not enabled when turning on metadata_csum

Warn the user if we're trying to enable metadata_csum on a FS that
doesn't support extents (since block maps cannot contain checksums).

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

misc/tune2fs.c

@@ -1113,6 +1113,14 @@ static int update_feature_set(ext2_filsys fs, char *features)
 		if (mount_flags & EXT2_MF_MOUNTED)
 			fputs(_("Cannot enable metadata_csum on a mounted "
 				"filesystem!\n"), stderr);
+		if (!EXT2_HAS_INCOMPAT_FEATURE(fs->super,
+				EXT3_FEATURE_INCOMPAT_EXTENTS))
+			printf("%s",
+			       _("Extents are not enabled.  The file extent "
+				 "tree can be checksummed, whereas block maps "
+				 "cannot.  Not enabling extents reduces the "
+				 "coverage of metadata checksumming.  "
+				 "Re-run with -O extent to rectify.\n"));
 		rewrite_checksums = 1;
 		/* metadata_csum supersedes uninit_bg */
 		fs->super->s_feature_ro_compat &=