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

It was reported that Commit 299603e8
("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>