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

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: 91581c4b3f29 ("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>