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

net/ipv6/ipv6_sockglue.c

@@ -503,7 +503,7 @@ static int do_ipv6_setsockopt(struct sock *sk, int level, int optname,
 			goto e_inval;
 		if (val > 255 || val < -1)
 			goto e_inval;
-		np->mcast_hops = val;
+		np->mcast_hops = (val == -1 ? IPV6_DEFAULT_MCASTHOPS : val);
 		retv = 0;
 		break;