ipv6: use ipv6_addr_scope() helper

ipv6_addr_type(&addr)&IPV6_ADDR_SCOPE_MASK could be replaced
by ipv6_addr_scope(), which is slightly faster.

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