ipv6: Fix BUG when disabled ipv6 module is unloaded

Do not try to "uninitialize" ipv6 if its initialization had been skipped
because module parameter disable=1 had been specified.
Reported-by: NThomas Backlund <tmb@mandriva.org>
Signed-off-by: NJohn Dykstra <john.dykstra1@gmail.com>
Acked-by: NBrian Haley <brian.haley@hp.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/af_inet6.c

@@ -1192,6 +1192,9 @@ module_init(inet6_init);
 
 static void __exit inet6_exit(void)
 {
+	if (disable_ipv6)
+		return;
+
 	/* First of all disallow new sockets creation. */
 	sock_unregister(PF_INET6);
 	/* Disallow any further netlink messages */