rcuperf: Add ability to performance-test call_rcu() and friends

This commit upgrades rcuperf so that it can do performance testing on
asynchronous grace-period primitives such as call_srcu().  There is
a new rcuperf.gp_async module parameter that specifies this new behavior,
with the pre-existing rcuperf.gp_exp testing expedited grace periods such as
synchronize_rcu_expedited, and with the default being to test synchronous
non-expedited grace periods such as synchronize_rcu().

There is also a new rcuperf.gp_async_max module parameter that specifies
the maximum number of outstanding callbacks per writer kthread, defaulting
to 1,000.  When this limit is exceeded, the writer thread invokes the
appropriate flavor of rcu_barrier() to wait for callbacks to drain.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Removed the redundant initialization noted by Arnd Bergmann. ]

Documentation/admin-guide/kernel-parameters.txt

@@ -3319,6 +3319,17 @@
 			This wake_up() will be accompanied by a
 			WARN_ONCE() splat and an ftrace_dump().
 
+	rcuperf.gp_async= [KNL]
+			Measure performance of asynchronous
+			grace-period primitives such as call_rcu().
+
+	rcuperf.gp_async_max= [KNL]
+			Specify the maximum number of outstanding
+			callbacks per writer thread.  When a writer
+			thread exceeds this limit, it invokes the
+			corresponding flavor of rcu_barrier() to allow
+			previously posted callbacks to drain.
+
 	rcuperf.gp_exp= [KNL]
 			Measure performance of expedited synchronous
 			grace-period primitives.