pkt_sched: remove unnecessary xchg() in packet classifiers

The use of xchg() hasn't been necessary since 2.2.something when proper
locking was added to packet schedulers. In the case of classifiers they
mostly weren't even necessary before that since they're mainly used
to assign a NULL pointer to the filter root in the ->destroy path;
the root is destroyed immediately after that.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/sched/cls_route.c b/net/sched/cls_route.c
index e3d8455..bdf1f41 100644
--- a/net/sched/cls_route.c
+++ b/net/sched/cls_route.c
@@ -260,7 +260,7 @@
 
 static void route4_destroy(struct tcf_proto *tp)
 {
-	struct route4_head *head = xchg(&tp->root, NULL);
+	struct route4_head *head = tp->root;
 	int h1, h2;
 
 	if (head == NULL)