gpiolib: cdev: Add INIT_KFIFO() for linereq events

The initialisation of the linereq events kfifo relies on the struct being
zeroed and a subsequent call to kfifo_alloc().  The call to kfifo_alloc()
is deferred until edge detection is first enabled for the linereq.  If the
kfifo is inadvertently accessed before the call to kfifo_alloc(), as was
the case in a recently discovered bug, it behaves as a FIFO of size 1 with
an element size of 0, so writes and reads to the kfifo appear successful
but copy no actual data.

As a defensive measure, initialise the kfifo with INIT_KFIFO() when the
events kfifo is constructed.  This initialises the kfifo element size
and zeroes its data pointer, so any inadvertant access prior to the
kfifo_alloc() call will trigger an oops.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Link: https://lore.kernel.org/r/20240529131953.195777-2-warthog618@gmail.com


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>