gpio: virtuser: Fix uninitialized data bug in gpio_virtuser_direction_do_write()

If *ppos is non-zero (user-space write split over multiple calls to
write()) then simple_write_to_buffer() won't initialize the start of the
buffer. Really, non-zero values for *ppos aren't going to work at all.
Check for that and return -EINVAL at the start of the function.

Fixes: 91581c4b3f ("gpio: virtuser: new virtual testing driver for the GPIO API")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Link: https://patch.msgid.link/ahP3BJWWy-m_qI0X@stanley.mountain
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>

drivers/gpio/gpio-virtuser.c

@@ -397,7 +397,7 @@ static ssize_t gpio_virtuser_direction_do_write(struct file *file,
 	char buf[32], *trimmed;
 	int ret, dir, val = 0;
 
-	if (count >= sizeof(buf))
+	if (*ppos != 0 || count >= sizeof(buf))
 		return -EINVAL;
 
 	ret = simple_write_to_buffer(buf, sizeof(buf) - 1, ppos, user_buf, count);
@@ -622,7 +622,7 @@ static ssize_t gpio_virtuser_consumer_write(struct file *file,
 	char buf[GPIO_VIRTUSER_NAME_BUF_LEN + 2];
 	int ret;
 
-	if (count >= sizeof(buf))
+	if (*ppos != 0 || count >= sizeof(buf))
 		return -EINVAL;
 
 	ret = simple_write_to_buffer(buf, GPIO_VIRTUSER_NAME_BUF_LEN, ppos,