locktorture: Support mutexes

Add a "mutex_lock" torture test. The main difference with the already
existing spinlock tests is that the latency of the critical region
is much larger. We randomly delay for (arbitrarily) either 500 ms or,
otherwise, 25 ms. While this can considerably reduce the amount of
writes compared to non blocking locks, if run long enough it can have
the same torturous effect. Furthermore it is more representative of
mutex hold times and can stress better things like thrashing.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Documentation/locking/locktorture.txt

@@ -40,6 +40,8 @@ torture_type	  Type of lock to torture. By default, only spinlocks will
 		     o "spin_lock_irq": spin_lock_irq() and spin_unlock_irq()
 					pairs.
 
+		     o "mutex_lock": mutex_lock() and mutex_unlock() pairs.
+
 torture_runnable  Start locktorture at boot time in the case where the
 		  module is built into the kernel, otherwise wait for
 		  torture_runnable to be set via sysfs before starting.