ext2: avoid unnecessary operation in ext2_error()

If filesystem has already mounted as read-only, then we don't have
to do it again.
Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/ext2/super.c

@@ -73,7 +73,7 @@ void ext2_error(struct super_block *sb, const char *function,
 
 	if (test_opt(sb, ERRORS_PANIC))
 		panic("EXT2-fs: panic from previous error\n");
-	if (test_opt(sb, ERRORS_RO)) {
+	if (!sb_rdonly(sb) && test_opt(sb, ERRORS_RO)) {
 		ext2_msg(sb, KERN_CRIT,
 			     "error: remounting filesystem read-only");
 		sb->s_flags |= SB_RDONLY;