Merge branch 'udp-msg_zerocopy'

Willem de Bruijn says:

====================
udp msg_zerocopy

Enable MSG_ZEROCOPY for udp sockets

Patch 1/3 is the main patch, a rework of RFC patch
  http://patchwork.ozlabs.org/patch/899630/
  more details in the patch commit message

Patch 2/3 is an optimization to remove a branch from the UDP hot path
  and refcount_inc/refcount_dec_and_test pair when zerocopy is used.
  This used to be included in the first patch in v2.

Patch 3/3 runs the already existing udp zerocopy tests
  as part of kselftest

See also recent Linux Plumbers presentation
  https://linuxplumbersconf.org/event/2/contributions/106/attachments/104/128/willemdebruijn-lpc2018-udpgso-presentation-20181113.pdf

Changes:
  v1 -> v2
    - Fixup reverse christmas tree violation
  v2 -> v3
    - Split refcount avoidance optimization into separate patch
      - Fix refcount leak on error in fragmented case
        (thanks to Paolo Abeni for pointing this one out!)
      - Fix refcount inc on zero
  v3 -> v4
    - Move skb_zcopy_set below the only kfree_skb that might cause
      a premature uarg destroy before skb_zerocopy_put_abort
      - Move the entire skb_shinfo assignment block, to keep that
	cacheline access in one place
====================

Signed-off-by: David S. Miller <davem@davemloft.net>