[NETFILTER] PPTP helper: fix PNS-PAC expectation call id

The reply tuple of the PNS->PAC expectation was using the wrong call id.

So we had the following situation:
- PNS behind NAT firewall
- PNS call id requires NATing
- PNS->PAC gre packet arrives first

then the PNS->PAC expectation is matched, and the other expectation
is deleted, but the PAC->PNS gre packets do not match the gre conntrack
because the call id is wrong.

We also cannot use ip_nat_follow_master().

Signed-off-by: Philip Craig <philipc@snapgear.com>
Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>