ipv6: use ipv6_addr_any() helper

ipv6_addr_any() is a faster way to determine if an addr
is ipv6 any addr, no need to compute the addr type.

Cc: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>