Merge branch 'ip_cmsg_csum'

Tom Herbert says:

====================
ip: Support checksum returned in csmg

This patch set allows the packet checksum for a datagram socket
to be returned in csum data in recvmsg. This allows userspace
to implement its own checksum over the data, for instance if an
IP tunnel was be implemented in user space, the inner checksum
could be validated.

Changes in this patch set:
  - Move checksum conversion to inet_sock from udp_sock. This
    generalizes checksum conversion for use with other protocols.
  - Move IP cmsg constants to a header file and make processing
    of the flags more efficient in ip_cmsg_recv
  - Return checksum value in cmsg. This is specifically the unfolded
    32 bit checksum of the full packet starting from the first byte
    returned in recvmsg

Tested: Wrote a little server to get checksums in cmsg for UDP and
        verfied correct checksum is returned.
====================

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