net: Save software checksum complete

In skb_checksum complete, if we need to compute the checksum for the
packet (via skb_checksum) save the result as CHECKSUM_COMPLETE.
Subsequent checksum verification can use this.

Also, added csum_complete_sw flag to distinguish between software and
hardware generated checksum complete, we should always be able to trust
the software computation.

Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/linux/skbuff.h
Simple merge

diff --cc net/core/datagram.c
Simple merge

diff --cc net/ipv4/gre_offload.c
Simple merge

diff --cc net/sunrpc/socklib.c
Simple merge