net: change skb->mac_header when Generic XDP calls adjust_head

Since XDP's view of the packet includes the MAC header, moving the start-
 of-packet with bpf_xdp_adjust_head needs to also update the offset of the
 MAC header (which is relative to skb->head, not to the skb->data that was
 changed).
Without this, tcpdump sees packets starting from the old MAC header rather
 than the new one, at least in my tests on the loopback device.

Fixes: b5cdae32 ("net: Generic XDP")
Signed-off-by: NEdward Cree <ecree@solarflare.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -3892,6 +3892,7 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
 		__skb_pull(skb, off);
 	else if (off < 0)
 		__skb_push(skb, -off);
+	skb->mac_header += off;
 
 	switch (act) {
 	case XDP_REDIRECT: