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/sched.h b/include/linux/sched.h
index 4059c0f..b9216eb 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1405,7 +1405,7 @@
 	int (*notifier)(void *priv);
 	void *notifier_data;
 	sigset_t *notifier_mask;
-	struct hlist_head task_works;
+	void *task_works;
 
 	struct audit_context *audit_context;
 #ifdef CONFIG_AUDITSYSCALL