rbd-nbd: fix stuck with disable request

rbd-nbd: fix stuck with disable request

Problem:
-------
Trying to disable any feature on an rbd image mapped with nbd leads to stuck
in rbd-nbd.

The rbd-nbd registers a watcher callback to detect image resize in
NBDWatchCtx::handle_notify(). The handle_notify calls image info method, which
calls refresh_if_required and it got stuck there.

It is getting stuck in ImageState::refresh_if_required() because
DisableFeaturesRequest issues update notifications while still holding onto
the exclusive lock with everything that has to do with it blocked.

Solution:
--------
Set only notify flag as part of NBDWatchCtx::handle_notify() and handle
the resize detection part as part of a different thread.

Fixes: https://tracker.ceph.com/issues/58740
Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>