[IPV6]: Use XOR and OR rather than mutiple ands for ipv6 address comparisons.

ipv6_addr_equal(), ipv6_addr_v4mapped(),
ipv6_addr_is_ll_all_{nodes,routers}(),
ipv6_masked_addr_cmp()

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>