[NETFILTER]: nf_conntrack_pptp: fix NAT setup of expected GRE connections

When an expected connection arrives, the NAT helper should be called to
set up NAT similar to the master connection. The PPTP conntrack helper
incorrectly checks whether the _expected_ connection has NAT setup before
calling the NAT helper (which is never the case), instead of checkeing
whether the _master_ connection is NATed.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>