virtio-net: restrict build_skb() use to some arches

build_skb() is supposed to be followed by
skb_reserve(skb, NET_IP_ALIGN), so that IP headers are word-aligned.
(Best practice is to reserve NET_IP_ALIGN+NET_SKB_PAD, but the NET_SKB_PAD
part is only a performance optimization if tunnel encaps are added.)

Unfortunately virtio_net has not provisioned this reserve.
We can only use build_skb() for arches where NET_IP_ALIGN == 0

We might refine this later, with enough testing.

Fixes: fb32856b ("virtio-net: page_to_skb() use build_skb when there's sufficient tailroom")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: virtualization@lists.linux-foundation.org
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/virtio_net.c

@@ -415,7 +415,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
 
 	shinfo_size = SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
 
-	if (len > GOOD_COPY_LEN && tailroom >= shinfo_size) {
+	if (!NET_IP_ALIGN && len > GOOD_COPY_LEN && tailroom >= shinfo_size) {
 		skb = build_skb(p, truesize);
 		if (unlikely(!skb))
 			return NULL;