pkt_sched: remove unnecessary xchg() in packet schedulers

The use of xchg() hasn't been necessary since 2.2.something when proper
locking was added to packet schedulers.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/sched/sch_atm.c b/net/sched/sch_atm.c
index ca90f6e..2a8b83a 100644
--- a/net/sched/sch_atm.c
+++ b/net/sched/sch_atm.c
@@ -102,7 +102,8 @@
 		return -EINVAL;
 	if (!new)
 		new = &noop_qdisc;
-	*old = xchg(&flow->q, new);
+	*old = flow->q;
+	flow->q = new;
 	if (*old)
 		qdisc_reset(*old);
 	return 0;