rcu: Adjust the comment of function rcu_is_watching

Because RCU avoids interrupting idle CPUs, rcu_is_watching() is used to
test whether or not it is currently legal to run RCU read-side critical
sections on this CPU.  However, the first sentence and last sentences
of current comment for rcu_is_watching have opposite meaning of what
is expected.  This commit therefore fixes this header comment.
Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

kernel/rcu/tree.c

@@ -904,12 +904,12 @@ void rcu_irq_enter_irqson(void)
 }
 
 /**
- * rcu_is_watching - see if RCU thinks that the current CPU is idle
+ * rcu_is_watching - see if RCU thinks that the current CPU is not idle
  *
  * Return true if RCU is watching the running CPU, which means that this
  * CPU can safely enter RCU read-side critical sections.  In other words,
- * if the current CPU is in its idle loop and is neither in an interrupt
- * or NMI handler, return true.
+ * if the current CPU is not in its idle loop or is in an interrupt or
+ * NMI handler, return true.
  */
 bool notrace rcu_is_watching(void)
 {