[NETFILTER]: nf_nat_sip: only perform RTP DNAT if SIP session was SNATed

DNAT of the the RTP session is only necessary if the SIP session has
been SNATed.

Signed-off-by: Jerome Borsboom <j.borsboom@erasmusmc.nl>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>