gre: When GRE csum is present count as encap layer wrt csum

In GRE demux if the GRE checksum pop rcv encapsulation so that any
encapsulated checksums are treated as tunnel checksums.
Signed-off-by: NTom Herbert <therbert@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/gre_demux.c

@@ -125,6 +125,7 @@ static int parse_gre_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
 			*csum_err = true;
 			return -EINVAL;
 		}
+		skb_pop_rcv_encapsulation(skb);
 		options++;
 	}