dissector: do not set invalid PPP protocol

The following flower filter fails to match non-PPP_IP{V6} packets
wrapped in PPP_SES protocol:

tc filter add dev eth0 ingress protocol ppp_ses flower \
        action simple sdata hi64

The reason is that proto local variable is being set even when
FLOW_DISSECT_RET_OUT_BAD status is returned.

The fix is to avoid setting proto variable if the PPP protocol is unknown.
Signed-off-by: NBoris Sukholitko <boris.sukholitko@broadcom.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/flow_dissector.c

@@ -1196,9 +1196,8 @@ bool __skb_flow_dissect(const struct net *net,
 			break;
 		}
 
-		proto = hdr->proto;
 		nhoff += PPPOE_SES_HLEN;
-		switch (proto) {
+		switch (hdr->proto) {
 		case htons(PPP_IP):
 			proto = htons(ETH_P_IP);
 			fdret = FLOW_DISSECT_RET_PROTO_AGAIN;