switch io_msg_ring() to CLASS(fd)

Use CLASS(fd) to get the file for sync message ring requests, rather
than open-code the file retrieval dance.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Link: https://lore.kernel.org/r/20241115034902.GP3387508@ZenIV
[axboe: make a more coherent commit message]
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: Al Viro

io_uring/msg_ring.c

@@ -333,7 +333,6 @@ int io_msg_ring(struct io_kiocb *req, unsigned int issue_flags)
 int io_uring_sync_msg_ring(struct io_uring_sqe *sqe)
 {
 	struct io_msg io_msg = { };
-	struct fd f;
 	int ret;
 
 	ret = __io_msg_ring_prep(&io_msg, sqe);
@@ -347,16 +346,13 @@ int io_uring_sync_msg_ring(struct io_uring_sqe *sqe)
 	if (io_msg.cmd != IORING_MSG_DATA)
 		return -EINVAL;
 
-	ret = -EBADF;
-	f = fdget(sqe->fd);
-	if (fd_file(f)) {
-		ret = -EBADFD;
-		if (io_is_uring_fops(fd_file(f)))
-			ret = __io_msg_ring_data(fd_file(f)->private_data,
-						 &io_msg, IO_URING_F_UNLOCKED);
-		fdput(f);
-	}
-	return ret;
+	CLASS(fd, f)(sqe->fd);
+	if (fd_empty(f))
+		return -EBADF;
+	if (!io_is_uring_fops(fd_file(f)))
+		return -EBADFD;
+	return  __io_msg_ring_data(fd_file(f)->private_data,
+				   &io_msg, IO_URING_F_UNLOCKED);
 }
 
 void io_msg_cache_free(const void *entry)