staging: pi433: fix memory leak with using debugfs_lookup()

When calling debugfs_lookup() the result must have dput() called on it,
otherwise the memory will leak over time. To make things simpler, just
call debugfs_lookup_and_remove() instead which handles all of the logic
at once. This requires saving off the root directory dentry to make
creation of individual device subdirectories easier.

Cc: Paulo Miguel Almeida <[e-mail]>
Cc: Dan Carpenter <[e-mail]>
Cc: Sidong Yang <[e-mail]>
Cc: Liu Shixin <[e-mail]>
Cc: "Uwe Kleine-König" <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/pi433/pi433_if.c

···
 static dev_t pi433_dev;
 static DEFINE_IDR(pi433_idr);
 static DEFINE_MUTEX(minor_lock); /* Protect idr accesses */
+static struct dentry *root_dir;	/* debugfs root directory for the driver */
 
 static struct class *pi433_class; /* mainly for udev to create /dev/pi433 */
 
···
 	/* spi setup */
 	spi_set_drvdata(spi, device);
 
-	entry = debugfs_create_dir(dev_name(device->dev),
-				   debugfs_lookup(KBUILD_MODNAME, NULL));
+	entry = debugfs_create_dir(dev_name(device->dev), root_dir);
 	debugfs_create_file("regs", 0400, entry, device, &pi433_debugfs_regs_fops);
 
 	return 0;
···
 static void pi433_remove(struct spi_device *spi)
 {
 	struct pi433_device	*device = spi_get_drvdata(spi);
-	struct dentry *mod_entry = debugfs_lookup(KBUILD_MODNAME, NULL);
 
-	debugfs_remove(debugfs_lookup(dev_name(device->dev), mod_entry));
+	debugfs_lookup_and_remove(dev_name(device->dev), root_dir);
 
 	/* free GPIOs */
 	free_gpio(device);
···
 		return PTR_ERR(pi433_class);
 	}
 
-	debugfs_create_dir(KBUILD_MODNAME, NULL);
+	root_dir = debugfs_create_dir(KBUILD_MODNAME, NULL);
 
 	status = spi_register_driver(&pi433_spi_driver);
 	if (status < 0) {
···
 	spi_unregister_driver(&pi433_spi_driver);
 	class_destroy(pi433_class);
 	unregister_chrdev(MAJOR(pi433_dev), pi433_spi_driver.driver.name);
-	debugfs_remove_recursive(debugfs_lookup(KBUILD_MODNAME, NULL));
+	debugfs_remove(root_dir);
 }
 module_exit(pi433_exit);