rcutorture: Abstract torture_stop_kthread()

Stopping of kthreads is not RCU-specific, so this commit abstracts
out torture_stop_kthread(), saving a few lines of code in the process.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

include/linux/torture.h

@@ -91,9 +91,12 @@ bool torture_must_stop_irq(void);
 void torture_kthread_stopping(char *title);
 int _torture_create_kthread(int (*fn)(void *arg), void *arg, char *s, char *m,
 			     char *f, struct task_struct **tp);
+void _torture_stop_kthread(char *m, struct task_struct **tp);
 
 #define torture_create_kthread(n, arg, tp) \
 	_torture_create_kthread(n, (arg), #n, "Creating " #n " task", \
 				"Failed to create " #n, &(tp))
+#define torture_stop_kthread(n, tp) \
+	_torture_stop_kthread("Stopping " #n " task", &(tp))
 
 #endif /* __LINUX_TORTURE_H */