tcp: enable MSG_ZEROCOPY

Enable support for MSG_ZEROCOPY to the TCP stack. TSO and GSO are
both supported. Only data sent to remote destinations is sent without
copying. Packets looped onto a local destination have their payload
copied to avoid unbounded latency.

Tested:
  A 10x TCP_STREAM between two hosts showed a reduction in netserver
  process cycles by up to 70%, depending on packet size. Systemwide,
  savings are of course much less pronounced, at up to 20% best case.

  msg_zerocopy.sh 4 tcp:

  without zerocopy
    tx=121792 (7600 MB) txc=0 zc=n
    rx=60458 (7600 MB)

  with zerocopy
    tx=286257 (17863 MB) txc=286257 zc=y
    rx=140022 (17863 MB)

  This test opens a pair of sockets over veth, one one calls send with
  64KB and optionally MSG_ZEROCOPY and on the other reads the initial
  bytes. The receiver truncates, so this is strictly an upper bound on
  what is achievable. It is more representative of sending data out of
  a physical NIC (when payload is not touched, either).

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>