ipv6: Avoid going through ->sk_net to access the netns

There is no need to go through sk->sk_net to access the net namespace
and its sysctl variables because we allocate the sock and initialize
sk_net just a few lines earlier in the same routine.

Signed-off-by: Jakub Sitnicki <jkbs@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>