udp: Generic functions to set checksum

Added udp_set_csum and udp6_set_csum functions to set UDP checksums
in packets. These are for simple UDP packets such as those that might
be created in UDP tunnels.

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

diff --cc include/net/ip6_checksum.h
Simple merge

diff --cc include/net/udp.h
Simple merge

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

diff --cc net/ipv6/ip6_checksum.c
Simple merge