net/ipv6/ipv6_sockglue.c: Removed redundant extern

extern int sysctl_mld_max_msf is already defined in linux/ipv6.h.
Signed-off-by: Eldad Zack <eldad@fogrefinery.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ipv6_sockglue.c

@@ -678,7 +678,6 @@ static int do_ipv6_setsockopt(struct sock *sk, int level, int optname,
 	}
 	case MCAST_MSFILTER:
 	{
-		extern int sysctl_mld_max_msf;
 		struct group_filter *gsf;
 
 		if (optlen < GROUP_FILTER_SIZE(0))