Merge branch 'udp-peek'

Merge branch 'udp-peek'

Willem de Bruijn says:

====================
udp: support SO_PEEK_OFF

Support peeking at a non-zero offset for UDP sockets. Match the
existing behavior on Unix datagram sockets.

1/3 makes the sk_peek_offset functions safe to use outside locks
2/3 removes udp headers before enqueue, to simplify offset arithmetic
3/3 introduces SO_PEEK_OFFSET support, with Unix socket peek semantics.

Changes
  v1->v2
    - squash patches 3 and 4
====================

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