sch_cake: Use tc_skb_protocol() helper for getting packet protocol

We shouldn't be using skb->protocol directly as that will miss cases with
hardware-accelerated VLAN tags. Use the helper instead to get the right
protocol number.
Reported-by: NKevin Darbyshire-Bryant <kevin@darbyshire-bryant.me.uk>
Signed-off-by: NToke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/sch_cake.c

@@ -1519,7 +1519,7 @@ static u8 cake_handle_diffserv(struct sk_buff *skb, u16 wash)
 {
 	u8 dscp;
 
-	switch (skb->protocol) {
+	switch (tc_skb_protocol(skb)) {
 	case htons(ETH_P_IP):
 		dscp = ipv4_get_dsfield(ip_hdr(skb)) >> 2;
 		if (wash && dscp)