btrfs: add read-only check to sysfs handler of features

We don't want to trigger the change on a read-only filesystem, similar
to what the label handler does.
Signed-off-by: David Sterba <dsterba@suse.cz>

fs/btrfs/sysfs.c

@@ -120,6 +120,9 @@ static ssize_t btrfs_feature_attr_store(struct kobject *kobj,
 	if (!fs_info)
 		return -EPERM;
 
+	if (fs_info->sb->s_flags & MS_RDONLY)
+		return -EROFS;
+
 	ret = kstrtoul(skip_spaces(buf), 0, &val);
 	if (ret)
 		return ret;