rcutorture: Exercise ->call() from NMI context

call_rcu() and call_srcu() are now safe to invoke from NMI, but
rcutorture never does, leaving the deferral path untested.

Add an ->nmi_capable flag to rcu_torture_ops.  For flavors that set it,
arm a per-CPU hardware perf counter whose overflow handler submits a
callback via ->call().  The handler acts only when in_nmi(), so only a
genuine NMI exercises the deferral path.  One preallocated callback per
CPU is kept in flight, guarded by an atomic, to avoid allocating in NMI.
The counter uses a fixed sample period rather than a frequency: a
frequency-based event sets TICK_DEP_BIT_PERF_EVENTS and would pin the tick
for the whole run on NO_HZ_FULL kernels.

Report the count issued from NMI ("nmi-calls:") and the count invoked
("nmi-cbs:").  rcu_torture_cleanup() disables the counters and then calls
cb_barrier(), which drains every deferred callback, so the two counts must
then match; a mismatch fails the test.  This relies on
rcu_barrier()/srcu_barrier() flushing deferred callbacks, as added earlier
in the series.

Set ->nmi_capable on the NMI-safe flavors: rcu, srcu, srcud, and
tasks-tracing (call_srcu() under the hood).  Tasks and Tasks Rude are left
alone, as call_rcu_tasks_generic() is not yet NMI-safe.

Enabled by default; the nmi_calls parameter disables it, which helps rule
NMI handling in or out when triaging a failure.  Requires
CONFIG_PERF_EVENTS and a hardware PMU: without one nothing is issued from
NMI and the end-of-test check compares zero against zero, so a pass does
not by itself mean the path ran.  That is the case under kvm.sh, which
boots qemu with -cpu kvm64 and no vPMU; rcu_torture_nmi_cleanup() says so
on the console.

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

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