virtio_net: fix truesize underestimation

We must account in skb->truesize, the size of the fragments, not the
used part of them.

Doing this work is important to avoid unexpected OOM situations.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Rusty Russell <rusty@rustcorp.com.au>
CC: "Michael S. Tsirkin" <mst@redhat.com>
CC: virtualization@lists.linux-foundation.org
CC: Krishna Kumar <krkumar2@in.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/virtio_net.c

@@ -150,6 +150,7 @@ static void set_skb_frag(struct sk_buff *skb, struct page *page,
 
 	skb->data_len += size;
 	skb->len += size;
+	skb->truesize += PAGE_SIZE;
 	skb_shinfo(skb)->nr_frags++;
 	*len -= size;
 }
@@ -287,7 +288,6 @@ static void receive_buf(struct net_device *dev, void *buf, unsigned int len)
 	}
 
 	hdr = skb_vnet_hdr(skb);
-	skb->truesize += skb->data_len;
 
 	u64_stats_update_begin(&stats->syncp);
 	stats->rx_bytes += skb->len;