librbd: avoid calling ImageUpdateWatchers twice

Problem:
-------
DisableFeaturesRequest calls send_notify_update, which actually calls
ImageCtx::notify_update, which is currently:

  void ImageCtx::notify_update() {
    state->handle_update_notification();
    ImageWatcher<>::notify_header_update(md_ctx, header_oid);
  }

The state->handle_update_notification() will fire ImageUpdateWatchers
notifications but does not wait for it to complete and then it calls
ImageWatcher<>::notify_header_update, which will send the "header update"
notification for all image watchers, including itself, so it will receive
it itself and will fire ImageUpdateWatchers the second time interfering
with the first call.

Solution:
--------
we don't need state->handle_update_notification() in ImageCtx::notify_update()
as we are going to receive our own notification anyway, here we are just
dropping it.

Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>

src/librbd/ImageCtx.cc

@@ -956,12 +956,10 @@ librados::IoCtx duplicate_io_ctx(librados::IoCtx& io_ctx) {
   }
 
   void ImageCtx::notify_update() {
-    state->handle_update_notification();
     ImageWatcher<>::notify_header_update(md_ctx, header_oid);
   }
 
   void ImageCtx::notify_update(Context *on_finish) {
-    state->handle_update_notification();
     image_watcher->notify_header_update(on_finish);
   }