rbd: check for watchers before trimming an image on 'rbd rm'

rbd: check for watchers before trimming an image on 'rbd rm'

Check for watchers before trimming image data to try to avoid getting
into the following situation:

  - user does 'rbd rm' on a mapped image with an fs mounted from it
  - 'rbd rm' trims (removes) all image data, only header is left
  - 'rbd rm' tries to remove a header and fails because krbd has a
    watcher registered on the header
  - at this point image cannot be unmapped because of the mounted fs
  - fs cannot be unmounted because all its data and metadata is gone

Unfortunately, this fix doesn't make it impossible to happen (the
required atomicity isn't there), but it's a big improvement over the
status quo.

Fixes: http://tracker.ceph.com/issues/7076
Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>