rcuperf: Add writer_holdoff boot parameter

This commit adds a writer_holdoff boot parameter to rcuperf, which is
intended to be used to test Tree SRCU's auto-expediting.  This
boot parameter is in microseconds, and defaults to zero (that is,
disabled).  Set it to a bit larger than srcutree.exp_holdoff,
keeping the nanosecond/microsecond conversion, to force Tree SRCU
to auto-expedite more aggressively.

This commit also adds documentation for this parameter, and fixes some
alphabetization while in the neighborhood.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -3357,17 +3357,22 @@
 	rcuperf.perf_runnable= [BOOT]
 			Start rcuperf running at boot time.
 
+	rcuperf.perf_type= [KNL]
+			Specify the RCU implementation to test.
+
 	rcuperf.shutdown= [KNL]
 			Shut the system down after performance tests
 			complete.  This is useful for hands-off automated
 			testing.
 
-	rcuperf.perf_type= [KNL]
-			Specify the RCU implementation to test.
-
 	rcuperf.verbose= [KNL]
 			Enable additional printk() statements.
 
+	rcuperf.writer_holdoff= [KNL]
+			Write-side holdoff between grace periods,
+			in microseconds.  The default of zero says
+			no holdoff.
+
 	rcutorture.cbflood_inter_holdoff= [KNL]
 			Set holdoff time (jiffies) between successive
 			callback-flood tests.