ipv6: fix unappropriate errno returned for non-multicast address

We need to check the passed in multicast address and return
appropriate errno(EINVAL) if it is not valid. And it's no need
to walk through the ipv6_mc_list in this situation.

Signed-off-by: Li Wei <lw@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>