btrfs: fix comment its device list mutex not volume lock

We have killed volume mutex (commit: dccdb07
btrfs: kill btrfs_fs_info::volume_mutex). This a trival one seems to have
escaped.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -1129,7 +1129,6 @@ static struct btrfs_fs_devices *clone_fs_devices(struct btrfs_fs_devices *orig)
  mutex_lock(&orig->device_list_mutex);
  fs_devices->total_devices = orig->total_devices;
 
- /* We have held the volume lock, it is safe to get the devices. */
  list_for_each_entry(orig_dev, &orig->devices, dev_list) {
  struct rcu_string *name;