[NETFILTER]: nf_conntrack_sip: RTP routing optimization

Optimize call routing between NATed endpoints: when an external
registrar sends a media description that contains an existing RTP
expectation from a different SNATed connection, the gatekeeper
is trying to route the call directly between the two endpoints.

We assume both endpoints can reach each other directly and
"un-NAT" the addresses, which makes the media stream go between
the two endpoints directly.

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