netfilter: nf_ct_pptp: fix DNATed PPTP connection address translation

When both the server and the client are NATed, the set-link-info control
packet containing the peer's call-id field is not properly translated.

I have verified that it was working in 2.6.16.13 kernel previously but
due to rewrite, this scenario stopped working (Not knowing exact version
when it stopped working).

Signed-off-by: Sanket Shah <sanket.shah@elitecore.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>