vt: keyboard, use DECLARE_BITMAP for key_down

key_down is sued as a bitmap using test_bit, set_bit and similar.
So declare it using DECLARE_BITMAP to make it obvious even from the
declaration.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20201029113222.32640-8-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>