merge task_work and rcu_head, get rid of separate allocation for keyring case

task_work and rcu_head are identical now; merge them (calling the result
struct callback_head, rcu_head #define'd to it), kill separate allocation
in security/keys since we can just use cred->rcu now.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index d1dd547..814c9ef 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -781,7 +781,7 @@
 		wake_up(&desc->wait_for_threads);
 }
 
-static void irq_thread_dtor(struct task_work *unused)
+static void irq_thread_dtor(struct callback_head *unused)
 {
 	struct task_struct *tsk = current;
 	struct irq_desc *desc;
@@ -813,7 +813,7 @@
  */
 static int irq_thread(void *data)
 {
-	struct task_work on_exit_work;
+	struct callback_head on_exit_work;
 	static const struct sched_param param = {
 		.sched_priority = MAX_USER_RT_PRIO/2,
 	};