ipv6: Fix default multicast hops setting.

As per RFC 3493 the default multicast hops setting
for a socket should be "1" just like ipv4.

Ironically we have a IPV6_DEFAULT_MCASTHOPS macro
it just wasn't being used.

Reported-by: Elliot Hughes <enh@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>