io_uring: unify calling convention for async prep handling

Make them consistent in preparation for defining a req async prep
handler. The readv/writev requests share a prep handler, move it one
level down so the initial one is consistent with the others.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -4176,6 +4176,16 @@ static inline int io_rw_prep_async(struct io_kiocb *req, int rw)
 	return 0;
 }
 
+static int io_readv_prep_async(struct io_kiocb *req)
+{
+	return io_rw_prep_async(req, READ);
+}
+
+static int io_writev_prep_async(struct io_kiocb *req)
+{
+	return io_rw_prep_async(req, WRITE);
+}
+
 /*
  * This is our waitqueue callback handler, registered through __folio_lock_async()
  * when we initially tried to do the IO with the iocb armed our waitqueue.
@@ -8136,9 +8146,9 @@ static int io_req_prep_async(struct io_kiocb *req)
 
 	switch (req->opcode) {
 	case IORING_OP_READV:
-		return io_rw_prep_async(req, READ);
+		return io_readv_prep_async(req);
 	case IORING_OP_WRITEV:
-		return io_rw_prep_async(req, WRITE);
+		return io_writev_prep_async(req);
 	case IORING_OP_SENDMSG:
 		return io_sendmsg_prep_async(req);
 	case IORING_OP_RECVMSG: