[NETFILTER]: nf_nat_pptp: fix expectation removal

When removing the expectation for the opposite direction, the PPTP NAT
helper initializes the tuple for lookup with the addresses of the
opposite direction, which makes the lookup fail.

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