gpiolib: cdev: allocate linereq using kvzalloc()

The size of struct linereq may exceed a page, so allocate space for
it using kvzalloc() instead of kzalloc() to handle the case where
memory is heavily fragmented and kzalloc() cannot find a sufficient
contiguous region.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>