[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>
diff --git a/net/ipv4/netfilter/nf_nat_sip.c b/net/ipv4/netfilter/nf_nat_sip.c
index fac97cf..a32d746 100644
--- a/net/ipv4/netfilter/nf_nat_sip.c
+++ b/net/ipv4/netfilter/nf_nat_sip.c
@@ -260,7 +260,11 @@
 	DEBUGP("ip_nat_sdp():\n");
 
 	/* Connection will come from reply */
-	newip = ct->tuplehash[!dir].tuple.dst.u3.ip;
+	if (ct->tuplehash[dir].tuple.src.u3.ip ==
+	    ct->tuplehash[!dir].tuple.dst.u3.ip)
+		newip = exp->tuple.dst.u3.ip;
+	else
+		newip = ct->tuplehash[!dir].tuple.dst.u3.ip;
 
 	exp->saved_ip = exp->tuple.dst.u3.ip;
 	exp->tuple.dst.u3.ip = newip;