[NETFILTER]: nf_conntrack_sip: perform NAT after parsing

Perform NAT last after parsing the packet. This makes no difference
currently, but is needed when dealing with registrations to make
sure we seen the unNATed addresses.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>