ext4: disallow changing journal_csum option during remount

ext4 does not permit changing the metadata or journal checksum feature
flag while mounted.  Until we decide to support that, don't allow a
remount to change the journal_csum flag (right now we silently fail to
change anything).
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/super.c

@@ -4845,6 +4845,14 @@ static int ext4_remount(struct super_block *sb, int *flags, char *data)
 		goto restore_opts;
 	}
 
+	if ((old_opts.s_mount_opt & EXT4_MOUNT_JOURNAL_CHECKSUM) ^
+	    test_opt(sb, JOURNAL_CHECKSUM)) {
+		ext4_msg(sb, KERN_ERR, "changing journal_checksum "
+			 "during remount not supported");
+		err = -EINVAL;
+		goto restore_opts;
+	}
+
 	if (test_opt(sb, DATA_FLAGS) == EXT4_MOUNT_JOURNAL_DATA) {
 		if (test_opt2(sb, EXPLICIT_DELALLOC)) {
 			ext4_msg(sb, KERN_ERR, "can't mount with "