[IPV6]: Fix rtnetlink dump infinite loop

The recent change to netlink dump "done" callback handling broke IPv6
which played dirty tricks with the "done" callback.  This causes an
infinite loop during a dump.

The following patch fixes it.

This bug was reported by Jeff Garzik.
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -1701,10 +1701,8 @@ static void fib6_dump_end(struct netlink_callback *cb)
 		fib6_walker_unlink(w);
 		kfree(w);
 	}
-	if (cb->args[1]) {
-		cb->done = (void*)cb->args[1];
-		cb->args[1] = 0;
-	}
+	cb->done = (void*)cb->args[1];
+	cb->args[1] = 0;
 }
 
 static int fib6_dump_done(struct netlink_callback *cb)