vhost-net: flush batched before enabling notifications

Commit 8c2e6b26ffe2 ("vhost/net: Defer TX queue re-enable until after
sendmsg") tries to defer the notification enabling by moving the logic
out of the loop after the vhost_tx_batch() when nothing new is spotted.
This caused unexpected side effects as the new logic is reused for
several other error conditions.

A previous patch reverted 8c2e6b26ffe2. Now, bring the performance
back up by flushing batched buffers before enabling notifications.

Reported-by: Jon Kohler <[e-mail]>
Cc: [e-mail]
Fixes: 8c2e6b26ffe2 ("vhost/net: Defer TX queue re-enable until after sendmsg")
Signed-off-by: Jason Wang <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>
Message-Id: <[e-mail]>

drivers/vhost/net.c

···
 			break;
 		/* Nothing new?  Wait for eventfd to tell us they refilled. */
 		if (head == vq->num) {
+			/* Flush batched packets to handle pending RX
+			 * work (if busyloop_intr is set) and to avoid
+			 * unnecessary virtqueue kicks.
+			 */
+			vhost_tx_batch(net, nvq, sock, &msg);
 			if (unlikely(busyloop_intr)) {
 				vhost_poll_queue(&vq->poll);
 			} else if (unlikely(vhost_enable_notify(&net->dev,