ipv6: Cache net in ip6_output

Keep net in a local variable so I can use it in NF_HOOK_COND
when I pass struct net to all of the netfilter hooks.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>