rcu, debug: detect stalled grace periods

this is a diagnostic patch for Classic RCU.

The approach is to record a timestamp at the beginning
of the grace period (in rcu_start_batch()), then have
rcu_check_callbacks() complain if:

 1.	it is running on a CPU that has holding up grace periods for
 	a long time (say one second).  This will identify the culprit
 	assuming that the culprit has not disabled hardware irqs,
 	instruction execution, or some such.

 2.	it is running on a CPU that is not holding up grace periods,
 	but grace periods have been held up for an even longer time
 	(say two seconds).

It is enabled via the default-off CONFIG_DEBUG_RCU_STALL kernel parameter.

Rather than exponential backoff, it backs off to once per 30 seconds.
My feeling upon thinking on it was that if you have stalled RCU grace
periods for that long, a few extra printk() messages are probably the
least of your worries...

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Yinghai Lu <yhlu.kernel@gmail.com>
Cc: David Witbrodt <dawitbro@sbcglobal.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>