ipv6: Set mcast_hops to IPV6_DEFAULT_MCASTHOPS when -1 was given.

We need to set np->mcast_hops to it's default value at this moment
otherwise when we use it and found it's value is -1, the logic to
get default hop limit doesn't take multicast into account and will
return wrong hop limit(IPV6_DEFAULT_HOPLIMIT) which is for unicast.

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