gpio: mockup: add missing single_release()

When using single_open() for opening, single_release() should be
used instead of seq_release(), otherwise there is a memory leak.

Fixes: 2a9e27408e ("gpio: mockup: rework debugfs interface")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>

drivers/gpio/gpio-mockup.c

@@ -309,6 +309,7 @@ static const struct file_operations gpio_mockup_debugfs_ops = {
 	.read = gpio_mockup_debugfs_read,
 	.write = gpio_mockup_debugfs_write,
 	.llseek = no_llseek,
+	.release = single_release,
 };
 
 static void gpio_mockup_debugfs_setup(struct device *dev,