libnvme: Unset ctrlr on teardown.

This way we do not attempt to tear down more than once,
which triggers an assert in mutex destruction.

src/add-ons/kernel/drivers/disk/nvme/libnvme/nvme_qpair.c

@@ -961,6 +961,9 @@ fail:
 
 void nvme_qpair_destroy(struct nvme_qpair *qpair)
 {
+	if (!qpair->ctrlr)
+		return; // Not initialized.
+
 	if (nvme_qpair_is_admin_queue(qpair))
 		_nvme_qpair_admin_qpair_destroy(qpair);
 
@@ -978,6 +981,8 @@ void nvme_qpair_destroy(struct nvme_qpair *qpair)
 	}
 	nvme_request_pool_destroy(qpair);
 
+	qpair->ctrlr = NULL;
+
 	pthread_mutex_destroy(&qpair->lock);
 }