[bug] label cli hangs until balance is completed

[Anand Jain <Anand.Jain@oracle.com>]

  'btrfs fi label /btrfs' will hang until balance is completed.
  (and probably even set label would hang). This is because we
  are trying to hold volume_mutex lock which balance will hold
  during its tenure.

-------
static int btrfs_ioctl_get_fslabel(struct file *file, void __user *arg)
::
         mutex_lock(&root->fs_info->volume_mutex);
         ret = copy_to_user(arg, label, len);
         mutex_unlock(&root->fs_info->volume_mutex);
--------

  I doubt if get label would need such a heavy weight lock?
  Do we have any other lock which could fit better here ?
  Any comments ?

Thanks,  Anand