Input: keyboard - add locking around event handling

Keyboard input handler is multiplexing events form all keyboard-like
devices in the system. Because of that per-device lock provided by
input core is not enough to prevent clashes in ked_event() and we
need our own lock to ensure that only one thread at a time executing
kbd_event().

Signed-off-by: Dmitry Torokhov <dtor@mail.ru>