IPv6: use ipv6_addr_v4mapped()

Change udp6_portaddr_hash() to use ipv6_addr_v4mapped()
inline instead of ipv6_addr_type().

Signed-off-by: Brian Haley <brian.haley@hp.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>