fs: replace f_rcuhead with f_task_work

The naming is actively misleading since we switched to
SLAB_TYPESAFE_BY_RCU. rcu_head is #define callback_head. Use
callback_head directly and rename f_rcuhead to f_task_work.

Add comments in there to explain what it's used for.

Link: https://lore.kernel.org/r/20231130-vfs-files-fixes-v1-3-e73ca6f4ea83@kernel.orgReviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/file_table.c

@@ -407,7 +407,7 @@ static void delayed_fput(struct work_struct *unused)
 
 static void ____fput(struct callback_head *work)
 {
-	__fput(container_of(work, struct file, f_rcuhead));
+	__fput(container_of(work, struct file, f_task_work));
 }
 
 /*
@@ -438,8 +438,8 @@ void fput(struct file *file)
 			return;
 		}
 		if (likely(!in_interrupt() && !(task->flags & PF_KTHREAD))) {
-			init_task_work(&file->f_rcuhead, ____fput);
-			if (!task_work_add(task, &file->f_rcuhead, TWA_RESUME))
+			init_task_work(&file->f_task_work, ____fput);
+			if (!task_work_add(task, &file->f_task_work, TWA_RESUME))
 				return;
 			/*
 			 * After this task has run exit_task_work(),

include/linux/fs.h

@@ -991,8 +991,10 @@ static inline int ra_has_index(struct file_ra_state *ra, pgoff_t index)
  */
 struct file {
 	union {
+		/* fput() uses task work when closing and freeing file (default). */
+		struct callback_head 	f_task_work;
+		/* fput() must use workqueue (most kernel threads). */
 		struct llist_node	f_llist;
-		struct rcu_head 	f_rcuhead;
 		unsigned int 		f_iocb_flags;
 	};