ip6: leave room for link headers in UDP

UDP over IPv6 was not leaving space for link headers,
resulting in the ethernet header being placed in its own mbuf
at the front of the mbuf chain sent down to the NIC driver.
This is inefficient, in terms of allocating 2x as many
header mbufs as needed, and its also confusing for drivers
which may expect to find ether/ip/l4 headers together in the same
mbuf.

Reviewed by: glebius, rrs, tuexen
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D49840

This is a port of e6ccd7093618, which was done by Robert
Watson in 2004 for IP4