btrfs: add comment to struct btrfs_fs_info::dirty_cowonly_roots

Add a comment to struct btrfs_fs_info::dirty_cowonly_roots to mention
that struct btrfs_fs_info::trans_lock is the lock that protects that
list.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/fs.h

@@ -576,6 +576,7 @@ struct btrfs_fs_info {
 	s32 dirty_metadata_batch;
 	s32 delalloc_batch;
 
+	/* Protected by 'trans_lock'. */
 	struct list_head dirty_cowonly_roots;
 
 	struct btrfs_fs_devices *fs_devices;