ext4: return EROFS if device is r/o and journal replay is needed

commit 4753d8a2 upstream.

If the file system requires journal recovery, and the device is
read-ony, return EROFS to the mount system call.  This allows xfstests
generic/050 to pass.
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/ext4/super.c

@@ -4038,7 +4038,8 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	 */
 	if (!test_opt(sb, NOLOAD) &&
 	    EXT4_HAS_COMPAT_FEATURE(sb, EXT4_FEATURE_COMPAT_HAS_JOURNAL)) {
-		if (ext4_load_journal(sb, es, journal_devnum))
+		err = ext4_load_journal(sb, es, journal_devnum);
+		if (err)
 			goto failed_mount3;
 	} else if (test_opt(sb, NOLOAD) && !(sb->s_flags & MS_RDONLY) &&
 	      EXT4_HAS_INCOMPAT_FEATURE(sb, EXT4_FEATURE_INCOMPAT_RECOVER)) {