[NETFILTER]: Fix CID offset bug in PPTP NAT helper debug message

The recent (kernel 2.6.15.1) fix for PPTP NAT helper introduced a
bug - which only appears if DEBUGP is enabled though.

The calculation of the CID offset into a PPTP request struct is
not correct, so that at least not the correct CID is displayed
if DEBUGP is enabled.

This patch corrects CID offset calculation and introduces a #define
for that.

Signed-off-by: Holger Eitzenberger <heitzenberger@astaro.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>