netfilter: Fix removal of GRE expectation entries created by PPTP

The uninitialized tuple structure caused incorrect hash calculation
and the lookup failed.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=106441


Signed-off-by: Anthony Lineham <anthony.lineham@alliedtelesis.co.nz>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>