Input: keyboard - fix theoretical race on vt switch

A VT switch can theoretically change fg_console between

        vc = vc_cons[fg_console].d
and
        kbd = kbd_table + fg_console

Fix it by replacing the second fg_console with vc->vc_num.

Signed-off-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>