virtio_net: clarify tailroom logic

Make tailroom math follow same logic as everything else, subtracing
values in the order in which things are laid out in the buffer.
Tested-by: Corentin Noël <corentin.noel@collabora.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/net/virtio_net.c

@@ -406,12 +406,13 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
 	 * add_recvbuf_mergeable() + get_mergeable_buf_len()
 	 */
 	truesize = headroom ? PAGE_SIZE : truesize;
-	tailroom = truesize - len - headroom - (hdr_padded_len - hdr_len);
+	tailroom = truesize - headroom;
 	buf = p - headroom;
 
 	len -= hdr_len;
 	offset += hdr_padded_len;
 	p += hdr_padded_len;
+	tailroom -= hdr_padded_len + len;
 
 	shinfo_size = SKB_DATA_ALIGN(sizeof(struct skb_shared_info));