syzbot reported a lockdep issue caused by tcp mmap() support.

I implemented Andy Lutomirski nice suggestions to resolve the
issue and increase scalability as well.

First patch is adding a new setsockopt() operation and changes mmap()
behavior.

Second patch changes tcp_mmap reference program.

v2:
 Added a missing page align of zc->length in tcp_zerocopy_receive()
 Properly clear zc->recv_skip_hint in case user request was completed.

Eric Dumazet (2):
  tcp: add TCP_ZEROCOPY_RECEIVE support for zerocopy receive
  selftests: net: tcp_mmap must use TCP_ZEROCOPY_RECEIVE

 include/uapi/linux/tcp.h               |   8 ++
 net/ipv4/tcp.c                         | 189 +++++++++++++------------
 tools/testing/selftests/net/tcp_mmap.c |  63 +++++----
 3 files changed, 142 insertions(+), 118 deletions(-)

-- 
2.17.0.441.gb46fe60e1d-goog