sys/netinet6: Use atomic(9) for dad_failures counter

Replace counter(9) usage with more lightweight atomic(9) in the
code handling RFC 7217 SLAAC address generation.

Also, use u_int types with this. Leaving dad_failures local to
in6_get_stableifid() as a uint64_t to avoid changing the generated
addresses from previous code; this also gives some headroom for
future changes.

While here, moved some #include lines to adhere to style(9).

Reviewed by: glebius, jhibbits, jtl, zlei
Approved by: glebius, jtl, zlei
Differential Revision: https://reviews.freebsd.org/D52731