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>