rcu: Fix obsolete DYNTICK_IRQ_NONIDLE comment

This commit updates the DYNTICK_IRQ_NONIDLE header comment to remove
the obsolete commentary about unmatched rcu_irq_{enter,exit}().
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

kernel/rcu/rcu.h

@@ -30,7 +30,7 @@
 #define RCU_TRACE(stmt)
 #endif /* #else #ifdef CONFIG_RCU_TRACE */
 
-/* Offset to allow for unmatched rcu_irq_{enter,exit}(). */
+/* Offset to allow distinguishing irq vs. task-based idle entry/exit. */
 #define DYNTICK_IRQ_NONIDLE	((LONG_MAX / 2) + 1)