net: suppress warning of smp_processor_id use.

John Kacur pointed out that the get_cpu_var used in net/sched/sch_generic.c
would trigger warnings. This was happing on a statistic variable and
by a softirq which is bound to a single thread.

John sent a patch that used local_irq_save which is a little bit of
overkill. This version uses preempt disable, but we still need to create
a preempt_disable_rt API that is only activated when PREEMPT_RT is configured.
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

net/sched/sch_generic.c

@@ -97,7 +97,9 @@ static inline int handle_dev_cpu_collision(struct sk_buff *skb,
 		 * Another cpu is holding lock, requeue & delay xmits for
 		 * some time.
 		 */
+		preempt_disable(); /* FIXME: we need an _rt version of this */
 		__get_cpu_var(netdev_rx_stat).cpu_collision++;
+		preempt_enable();
 		ret = dev_requeue_skb(skb, q);
 	}