rcutorture: Add races with task-exit processing

Several variants of Linux-kernel RCU interact with task-exit processing,
including preemptible RCU, Tasks RCU, and Tasks Trace RCU.  This commit
therefore adds testing of this interaction to rcutorture by adding
rcutorture.read_exit_burst and rcutorture.read_exit_delay kernel-boot
parameters.  These kernel parameters control the frequency and spacing
of special read-then-exit kthreads that are spawned.

[ paulmck: Apply feedback from Dan Carpenter's static checker. ]
[ paulmck: Reduce latency to avoid false-positive shutdown hangs. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/admin-guide/kernel-parameters.txt

@@ -4258,6 +4258,20 @@
 			Set time (jiffies) between CPU-hotplug operations,
 			or zero to disable CPU-hotplug testing.
 
+	rcutorture.read_exit= [KNL]
+			Set the number of read-then-exit kthreads used
+			to test the interaction of RCU updaters and
+			task-exit processing.
+
+	rcutorture.read_exit_burst= [KNL]
+			The number of times in a given read-then-exit
+			episode that a set of read-then-exit kthreads
+			is spawned.
+
+	rcutorture.read_exit_delay= [KNL]
+			The delay, in seconds, between successive
+			read-then-exit testing episodes.
+
 	rcutorture.shuffle_interval= [KNL]
 			Set task-shuffle interval (s).  Shuffling tasks
 			allows some CPUs to go into dyntick-idle mode