list: Use WRITE_ONCE() when initializing list_head structures

Code that does lockless emptiness testing of non-RCU lists is relying
on INIT_LIST_HEAD() to write the list head's ->next pointer atomically,
particularly when INIT_LIST_HEAD() is invoked from list_del_init().
This commit therefore adds WRITE_ONCE() to this function's pointer stores
that could affect the head's ->next pointer.
Reported-by: NAndrey Konovalov <andreyknvl@google.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

include/linux/list.h

@@ -24,7 +24,7 @@
 
 static inline void INIT_LIST_HEAD(struct list_head *list)
 {
-	list->next = list;
+	WRITE_ONCE(list->next, list);
 	list->prev = list;
 }