Input: joydev - validate axis/button maps before clobbering current ones

Up to now axis and button map validation was done after the user-supplied
values were copied over the driver's map. This patch copies the
user-supplied values into temporary buffers and validated them before
overwriting the driver's permanent maps.

Also change JSIOCGBTNMAP and JSIOCGAXMAP to return number of bytes returned
to userspace instead of 0.

Signed-off-by: Stephen Kitt <steve@sk2.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>