rcu: Optimize cond_resched_rcu_qs()

The current implementation of cond_resched_rcu_qs() can invoke
rcu_note_voluntary_context_switch() twice in the should_resched()
case, once via the call to __schedule() and once directly.  However, as
noted by Joe Lawrence in a patch to the team subsystem, cond_resched()
returns an indication as to whether or not the call to __schedule()
actually happened.  This commit therefore changes cond_resched_rcu_qs()
so as to invoke rcu_note_voluntary_context_switch() only when __schedule()
was not called.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/linux/rcupdate.h

@@ -348,8 +348,8 @@ extern struct srcu_struct tasks_rcu_exit_srcu;
  */
 #define cond_resched_rcu_qs() \
 do { \
-	rcu_note_voluntary_context_switch(current); \
-	cond_resched(); \
+	if (!cond_resched()) \
+		rcu_note_voluntary_context_switch(current); \
 } while (0)
 
 #if defined(CONFIG_DEBUG_LOCK_ALLOC) || defined(CONFIG_RCU_TRACE) || defined(CONFIG_SMP)