new helper: skb_copy_and_csum_datagram_msg()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

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

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

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

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