nvme: correct locking around completion

nvme_poll_queues is already protected by q->lock, and
AIO callbacks are invoked outside the AioContext lock.
So remove the acquire/release pair in nvme_handle_event.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-Id: <20180814062739.19640-1-pbonzini@redhat.com>
Signed-off-by: NFam Zheng <famz@redhat.com>

block/nvme.c

@@ -489,10 +489,8 @@ static void nvme_handle_event(EventNotifier *n)
     BDRVNVMeState *s = container_of(n, BDRVNVMeState, irq_notifier);
 
     trace_nvme_handle_event(s);
-    aio_context_acquire(s->aio_context);
     event_notifier_test_and_clear(n);
     nvme_poll_queues(s);
-    aio_context_release(s->aio_context);
 }
 
 static bool nvme_add_io_queue(BlockDriverState *bs, Error **errp)