netlink: make netlink work correctly on CHERI.

Current Netlink message writer code relies on executing callbacks
with arbitrary data (pointer or integer) to flush the completed
messages.
This arbitrary data is stored as a union of { void *, uint64_t }.
At some stage, the message flushing code copied this data, using
direct uint64_t assignment instead of copying the union. It lead
to failure on CHERI, as sizeof(pointer) == 16 there.

Fix the code by making union non-anonymous and copying it entirely.

Reviewed by: br, jhb, jrtc27
Differential Revision: https://reviews.freebsd.org/D39557
MFC after: 2 weeks