trim task_work: get rid of hlist

layout based on Oleg's suggestion; single-linked list,
task->task_works points to the last element, forward pointer
from said last element points to head.  I'd still prefer
much more regular scheme with two pointers in task_work,
but...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/include/linux/task_work.h b/include/linux/task_work.h
index 627421c..3b3e2c8 100644
--- a/include/linux/task_work.h
+++ b/include/linux/task_work.h
@@ -8,7 +8,7 @@
 typedef void (*task_work_func_t)(struct task_work *);
 
 struct task_work {
-	struct hlist_node hlist;
+	struct task_work *next;
 	task_work_func_t func;
 };
 
@@ -24,7 +24,7 @@
 
 static inline void exit_task_work(struct task_struct *task)
 {
-	if (unlikely(!hlist_empty(&task->task_works)))
+	if (unlikely(task->task_works))
 		task_work_run();
 }