net-ipv6: Fix alleged compiler warning in ipv6_exthdrs_len()

It was reported that Commit 299603e8370a93dd5d8e8d800f0dff1ce2c53d36
("net-gro: Prepare GRO stack for the upcoming tunneling support")
triggered a compiler warning in ipv6_exthdrs_len():

net/ipv6/ip6_offload.c: In function ‘ipv6_gro_complete’:
net/ipv6/ip6_offload.c:178:24: warning: ‘optlen’ may be used uninitialized in this function [-Wmaybe-u
    opth = (void *)opth + optlen;
    			^
    net/ipv6/ip6_offload.c:164:22: note: ‘optlen’ was declared here
    int len = 0, proto, optlen;
                        ^
Note that there was no real bug here - optlen was never uninitialized
before use. (Was the version of gcc I used smarter to not complain?)

Reported-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: H.K. Jerry Chu <hkchu@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_offload.c

@@ -160,8 +160,8 @@ out:
 static int ipv6_exthdrs_len(struct ipv6hdr *iph,
 			    const struct net_offload **opps)
 {
-	struct ipv6_opt_hdr *opth = NULL;
-	int len = 0, optlen = 0, proto;
+	struct ipv6_opt_hdr *opth = (void *)iph;
+	int len = 0, proto, optlen = sizeof(*iph);
 
 	proto = iph->nexthdr;
 	for (;;) {
@@ -172,12 +172,8 @@ static int ipv6_exthdrs_len(struct ipv6hdr *iph,
 			if (!((*opps)->flags & INET6_PROTO_GSO_EXTHDR))
 				break;
 		}
-		if (opth == NULL) {
-			opth = (void *)(iph+1);
-		} else {
-			optlen = ipv6_optlen(opth);
-			opth = (void *)opth + optlen;
-		}
+		opth = (void *)opth + optlen;
+		optlen = ipv6_optlen(opth);
 		len += optlen;
 		proto = opth->nexthdr;
 	}