ipv6: fix checksum annotation in udp6_csum_init

Cc: Tom Herbert <tom@herbertland.com>
Fixes: 4068579e ("net: Implmement RFC 6936 (zero RX csums for UDP/IPv6")
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>