Input: add safety guards to input_set_keycode()

If we happen to have a garbage in input device's keycode table with values
too big we'll end up doing clear_bit() with offset way outside of our
bitmaps, damaging other objects within an input device or even outside of
it. Let's add sanity checks to the returned old keycodes.

Reported-by:  <syzbot+c769968809f9359b07aa@syzkaller.appspotmail.com>
Reported-by:  <syzbot+76f3a30e88d256644c78@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/r/20191207212757.GA245964@dtor-ws


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>