kernel-netlink: Copy current usage stats to new SA in update_sa()

kernel-netlink: Copy current usage stats to new SA in update_sa()

This is needed to fix usage stats sent via RADIUS Accounting if clients
use MOBIKE or e.g. the kernel notifies us about a changed NAT mapping.
The upper layers won't expect the stats to get reset if only the IPs have
changed (and some kernel interface might actually allow such updates
without reset).

It also fixes traffic based lifetimes in such situations.

Fixes #799.