[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>

net/ipv4/netfilter/ip_conntrack_helper_pptp.c

@@ -194,6 +194,7 @@ static void pptp_destroy_siblings(struct ip_conntrack *ct)
 {
 	struct ip_conntrack_tuple t;
 
+	ip_ct_gre_keymap_destroy(ct);
 	/* Since ct->sibling_list has literally rusted away in 2.6.11,
 	 * we now need another way to find out about our sibling
 	 * contrack and expects... -HW */