rbd: call rbd_dev_unprobe() after unwatching and flushing notifies

rbd_dev_unprobe() is supposed to undo most of rbd_dev_image_probe(),
including rbd_dev_header_info(), which means that rbd_dev_header_info()
isn't supposed to be called after rbd_dev_unprobe().

However, rbd_dev_image_release() calls rbd_dev_unprobe() before
rbd_unregister_watch().  This is racy because a header update notify
can sneak in:

  "rbd unmap" thread                   ceph-watch-notify worker

  rbd_dev_image_release()
    rbd_dev_unprobe()
      free and zero out header
                                       rbd_watch_cb()
                                         rbd_dev_refresh()
                                           rbd_dev_header_info()
                                             read in header

The same goes for "rbd map" because rbd_dev_image_probe() calls
rbd_dev_unprobe() on errors.  In both cases this results in a memory
leak.

Fixes: fd22aef8 ("rbd: move rbd_unregister_watch() call into rbd_dev_image_release()")
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>