netlabel: If PF_INET6, check sk_buff ip header version

When resolving a fallback label, check the sk_buff version as it
is possible (e.g. SCTP) to have family = PF_INET6 while
receiving ip_hdr(skb)->version = 4.

Signed-off-by: Richard Haines <richard_c_haines@btinternet.com>
Acked-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>