Merge branch 'udp-ipv6-optimisations'

Pavel Begunkov says:

====================
udp/ipv6 optimisations

Shed some weight from udp/ipv6. Zerocopy benchmarks over dummy showed
~5% tx/s improvement, should be similar for small payload non-zc
cases.

The performance comes from killing 4 atomics and a couple of big struct
memcpy/memset. 1/10 removes a pair of atomics on dst refcounting for
cork->skb setup, 9/10 saves another pair on cork init. 5/10 and 8/10
kill extra 88B memset and memcpy respectively.
====================

Link: https://lore.kernel.org/r/cover.1643243772.git.asml.silence@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>