ipv6: Fail with appropriate error code when setting not-applicable sockopt.

IPV6_MULTICAST_HOPS, for example, is not valid for stream sockets.
Since they are virtually unavailable for stream sockets,
we should return ENOPROTOOPT instead of EINVAL.

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