netfilter: use consistent ipv4 network offset in xt_AUDIT

Even though the skb->data pointer has been moved from the link layer
header to the network layer header, use the same method to calculate the
offset in ipv4 and ipv6 routines.

Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
[PM: munged subject line]
Signed-off-by: Paul Moore <paul@paul-moore.com>