[NETFILTER]: PPTP conntrack: fix another GRE keymap leak

When the master PPTP connection times out while still having unfullfilled
expectations (and a GRE keymap entry) associated with it, the keymap entry
is not destroyed.

Add a destroy callback to struct ip_conntrack_helper and use it to destroy
PPTP siblings when the master is destroyed.

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