virtio-net: avoid unnecessary sg initialzation

Usually an skb does not have up to MAX_SKB_FRAGS frags. So no need to
initialize the unuse part of sg. This patch initialize the sg based on
the real number it will used:

- during xmit, it could be inferred from nr_frags and can_push.
- for small receive buffer, it will also be 2.

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/virtio_net.c

@@ -540,7 +540,7 @@ static int add_recvbuf_small(struct virtnet_info *vi, struct receive_queue *rq,
 	skb_put(skb, GOOD_PACKET_LEN);
 
 	hdr = skb_vnet_hdr(skb);
-	sg_init_table(rq->sg, MAX_SKB_FRAGS + 2);
+	sg_init_table(rq->sg, 2);
 	sg_set_buf(rq->sg, hdr, vi->hdr_len);
 	skb_to_sgvec(skb, rq->sg + 1, 0, skb->len);
 
@@ -893,7 +893,7 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
 	if (vi->mergeable_rx_bufs)
 		hdr->num_buffers = 0;
 
-	sg_init_table(sq->sg, MAX_SKB_FRAGS + 2);
+	sg_init_table(sq->sg, skb_shinfo(skb)->nr_frags + (can_push ? 1 : 2));
 	if (can_push) {
 		__skb_push(skb, hdr_len);
 		num_sg = skb_to_sgvec(skb, sq->sg, 0, skb->len);