[NETFILTER]: PPTP conntrack: fix GRE keymap leak

When destroying the GRE expectations without having seen the GRE connection
the keymap entry is not freed, leading to a memory leak and, in case of
a following call within the same session, failure during expectation setup.

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