ipv6: Check IPV6_MULTICAST_LOOP option value.

Only 0 and 1 are valid for IPV6_MULTICAST_LOOP socket option,
and we should return an error of EINVAL otherwise, per RFC3493.

Based on patch from Shan Wei <shanwei@cn.fujitsu.com>.

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