rcutorture: Test RCU readers from hardware interrupt handlers

Although rcutorture has long had the irqreader module parameter, this
parameter results only in RCU readers in softirq handlers, specifically,
timers.  This commit therefore uses smp_call_function_single() to test
RCU readers in real hardware interrupt handlers, thus providing the full
effect from the irqreader module parameter.

However, consistency/debug checks must account for the possibility that
the smp_call_function_single() handler function is directly invoked
from the idle loop, in which case, for example, in_hardirq() will
return false.  This commit uses a per-CPU variable to record being in
the rcu_torture_irq() smp_call_function_single() handler function.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

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