vhost-net: check the support of mergeable buffer outside the receive loop

No need to check the support of mergeable buffer inside the recevie
loop as the whole handle_rx()_xx is in the read critical region.  So
this patch move it ahead of the receiving loop.
Signed-off-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

drivers/vhost/net.c

@@ -411,7 +411,7 @@ static void handle_rx_mergeable(struct vhost_net *net)
 		.hdr.gso_type = VIRTIO_NET_HDR_GSO_NONE
 	};
 	size_t total_len = 0;
-	int err, headcount;
+	int err, headcount, mergeable;
 	size_t vhost_hlen, sock_hlen;
 	size_t vhost_len, sock_len;
 	/* TODO: check that we are running from vhost_worker? */
@@ -427,6 +427,7 @@ static void handle_rx_mergeable(struct vhost_net *net)
 
 	vq_log = unlikely(vhost_has_feature(&net->dev, VHOST_F_LOG_ALL)) ?
 		vq->log : NULL;
+	mergeable = vhost_has_feature(&net->dev, VIRTIO_NET_F_MRG_RXBUF);
 
 	while ((sock_len = peek_head_len(sock->sk))) {
 		sock_len += sock_hlen;
@@ -476,7 +477,7 @@ static void handle_rx_mergeable(struct vhost_net *net)
 			break;
 		}
 		/* TODO: Should check and handle checksum. */
-		if (vhost_has_feature(&net->dev, VIRTIO_NET_F_MRG_RXBUF) &&
+		if (likely(mergeable) &&
 		    memcpy_toiovecend(vq->hdr, (unsigned char *)&headcount,
 				      offsetof(typeof(hdr), num_buffers),
 				      sizeof hdr.num_buffers)) {