rcu: Remove "select IRQ_WORK" from config TREE_RCU

The 48a7639c ("rcu: Make callers awaken grace-period kthread")
removed the irq_work_queue(), so the TREE_RCU doesn't need
irq work any more.  This commit therefore updates RCU's Kconfig and
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

init/Kconfig

@@ -470,7 +470,6 @@ choice
 config TREE_RCU
 	bool "Tree-based hierarchical RCU"
 	depends on !PREEMPT && SMP
-	select IRQ_WORK
 	help
 	  This option selects the RCU implementation that is
 	  designed for very large SMP system with hundreds or
@@ -480,7 +479,6 @@ config TREE_RCU
 config PREEMPT_RCU
 	bool "Preemptible tree-based hierarchical RCU"
 	depends on PREEMPT
-	select IRQ_WORK
 	help
 	  This option selects the RCU implementation that is
 	  designed for very large SMP systems with hundreds or

kernel/rcu/tree.h

@@ -27,7 +27,6 @@
 #include <linux/threads.h>
 #include <linux/cpumask.h>
 #include <linux/seqlock.h>
-#include <linux/irq_work.h>
 
 /*
  * Define shape of hierarchy based on NR_CPUS, CONFIG_RCU_FANOUT, and