XIA: add method ppal_destroy to struct xip_dst

It has been identified that principals occasionally need to
release resources attached to struct xip_dst, and
there was no easy way to do so.

This patch also adapts U4ID principal to use this new facility,
what fixs the following bug found by Cody Doucette:

There's only a struct dst_ops per struct net in XIA,
so, when u4id_deliver changed the destroy field of struct dst_ops,
it was forcing *every* XDST entry of that container to use
destroy_u4id_tunnel_dest.