child-sa: Handle refcount overflow for unique mark/if_id allocation gracefully

child-sa: Handle refcount overflow for unique mark/if_id allocation gracefully

The refcount_t for allocating unique marks and interface IDs may overflow or
hit the special value for unique marks/if_ids, in the worst case not setting it
on CHILD_SAs that should have one.

As (potentially two) marks/if_ids are allocated only for newly created CHILD_SAs,
but not for rekeying, this not very likely. Still, if a setup uses
aggressive re-authentication and or re-creates CHILD_SAs every minute,
a gateway with 100'000 tunnels may hit the overflow within a month uptime.