ipv6: Make ipv6_addr_is_XXX() return boolean.

ipv6_addr_is_{multicast,ll_all_nodes,ll_all_routers,isatap}()
return boolean.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>