macvtap: advance iov iterator when needed in macvtap_put_user()

When mergeable buffer is used, vnet_hdr_sz is greater than sizeof struct
virtio_net_hdr. So we need advance the iov iterators in this case.

Fixes 6c36d2e2 ("macvtap: Use iovec iterators")
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/macvtap.c

@@ -797,6 +797,8 @@ static ssize_t macvtap_put_user(struct macvtap_queue *q,
 		if (copy_to_iter(&vnet_hdr, sizeof(vnet_hdr), iter) !=
 		    sizeof(vnet_hdr))
 			return -EFAULT;
+
+		iov_iter_advance(iter, vnet_hdr_len - sizeof(vnet_hdr));
 	}
 	total = vnet_hdr_len;
 	total += skb->len;