IPv6: sync IP_NO_SND_TAG_RL support from IPv4

The IP_NO_SND_TAG_RL flag to ip{,6}_output() means that the packets
being sent should bypass hardware rate limiting. This is typically used
by modern TCP stacks for rexmits.

This support was added to IPv4 in r352657, but never added to IPv6, even
though rack and bbr call ip6_output() with this flag.

Reviewed by: rrs
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D24822