nbd/server: avoid per-NBDRequest nbd_client_get/put()

nbd/server: avoid per-NBDRequest nbd_client_get/put()

nbd_trip() processes a single NBD request from start to finish and holds
an NBDClient reference throughout. NBDRequest does not outlive the scope
of nbd_trip(). Therefore it is unnecessary to ref/unref NBDClient for
each NBDRequest.

Removing these nbd_client_get()/nbd_client_put() calls will make
thread-safety easier in the commits that follow.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20231221192452.1785567-5-stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>