udp: fix IP_CHECKSUM handling

udp: fix IP_CHECKSUM handling

First bug was added in commit ad6f939ab193 ("ip: Add offset parameter to
ip_cmsg_recv") : Tom missed that ipv4 udp messages could be received on
AF_INET6 socket. ip_cmsg_recv(msg, skb) should have been replaced by
ip_cmsg_recv_offset(msg, skb, sizeof(struct udphdr));

Then commit e6afc8ace6dd ("udp: remove headers from UDP packets before
queueing") forgot to adjust the offsets now UDP headers are pulled
before skb are put in receive queue.

Fixes: ad6f939ab193 ("ip: Add offset parameter to ip_cmsg_recv")
Fixes: e6afc8ace6dd ("udp: remove headers from UDP packets before queueing")
Signed-off-by: Eric Dumazet <[email protected]>
Cc: Sam Kumar <[email protected]>
Cc: Willem de Bruijn <[email protected]>
Tested-by: Willem de Bruijn <[email protected]>
Signed-off-by: David S. Miller <[email protected]>