io_uring: stop using io_wq_work as an fd placeholder

There are two reasons why this isn't the best idea:

- It's an odd area to grab a bit of storage space, hence it's an odd area
  to grab storage from.
- It puts the 3rd io_kiocb cacheline into the hot path, where normal hot
  path just needs the first two.

Use 'cflags' for joint fd/cflags storage. We only need fd until we
successfully issue, and we only need cflags once a request is done and is
completed.

Fixes: 6bf9c47a ("io_uring: defer file assignment")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io-wq.h

@@ -155,7 +155,6 @@ struct io_wq_work_node *wq_stack_extract(struct io_wq_work_node *stack)
 struct io_wq_work {
 	struct io_wq_work_node list;
 	unsigned flags;
-	int fd;
 };
 
 static inline struct io_wq_work *wq_next_work(struct io_wq_work *work)

fs/io_uring.c

@@ -907,7 +907,11 @@ struct io_kiocb {
 
 	u64				user_data;
 	u32				result;
-	u32				cflags;
+	/* fd initially, then cflags for completion */
+	union {
+		u32			cflags;
+		int			fd;
+	};
 
 	struct io_ring_ctx		*ctx;
 	struct task_struct		*task;
@@ -7090,9 +7094,9 @@ static bool io_assign_file(struct io_kiocb *req, unsigned int issue_flags)
 		return true;
 
 	if (req->flags & REQ_F_FIXED_FILE)
-		req->file = io_file_get_fixed(req, req->work.fd, issue_flags);
+		req->file = io_file_get_fixed(req, req->fd, issue_flags);
 	else
-		req->file = io_file_get_normal(req, req->work.fd);
+		req->file = io_file_get_normal(req, req->fd);
 	if (req->file)
 		return true;
 
@@ -7630,7 +7634,7 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
 	if (io_op_defs[opcode].needs_file) {
 		struct io_submit_state *state = &ctx->submit_state;
 
-		req->work.fd = READ_ONCE(sqe->fd);
+		req->fd = READ_ONCE(sqe->fd);
 
 		/*
 		 * Plug now if we have more than 2 IO left after this, and the