Turns out that the IPIP tunnel used by IPCOMP states are only freed
if the IPCOMP state is deleted by xfrm_state_delete.

This is not the case for all states.  For example, an immature IPCOMP
state that dies in add_sa will not go through xfrm_state_delete.

The following patch moves the delete_tunnel call into IPCOMP's
destructor.  I think it makes more sense there as IPCOMP is the
only user of the tunnel anyway.