bpf: fix virtio-net's length calc for XDP_PASS

In commit 6870de435b90 ("bpf: make virtio compatible w/
bpf_xdp_adjust_tail") i didn't account for vi->hdr_len during new
packet's length calculation after bpf_prog_run in receive_mergeable.
because of this all packets, if they were passed to the kernel,
were truncated by 12 bytes.

Fixes:6870de435b90 ("bpf: make virtio compatible w/ bpf_xdp_adjust_tail")
Reported-by: David Ahern <dsahern@gmail.com>

Signed-off-by: Nikita V. Shirokov <tehnerd@tehnerd.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

drivers/net/virtio_net.c

@@ -765,7 +765,7 @@ static struct sk_buff *receive_mergeable(struct net_device *dev,
 			/* recalculate len if xdp.data or xdp.data_end were
 			 * adjusted
 			 */
-			len = xdp.data_end - xdp.data;
+			len = xdp.data_end - xdp.data + vi->hdr_len;
 			/* We can only create skb based on xdp_page. */
 			if (unlikely(xdp_page != page)) {
 				rcu_read_unlock();