MFp4 bz_ipv6_fast:

Migrate udp6_send() v4mapped code to udp6_output() saving us a re-lock and
further simplifying the address-family handling code by eliminating
AF_INET checks and almost all v4mapped handling right after the start
as cases could actually not happen anymore.

Rework output path locking similar to UDP4 allowing for better
parallelism (see r222488, and later versions).

Sponsored by: The FreeBSD Foundation (2012)
Sponsored by: iXsystems (2012)
Differential Revision: https://reviews.freebsd.org/D3721