vt: fix unicode console freeing with a common interface

By directly using kfree() in different places we risk missing one if
it is switched to using vfree(), especially if the corresponding
vmalloc() is hidden away within a common abstraction.

Oh wait, that's exactly what happened here.

So let's fix this by creating a common abstraction for the free case
as well.

Signed-off-by: Nicolas Pitre <nico@fluxnic.net>
Reported-by:  <syzbot+0bfda3ade1ee9288a1be@syzkaller.appspotmail.com>
Fixes: 9a98e7a8

 ("vt: don't use kmalloc() for the unicode screen buffer")
Cc: <stable@vger.kernel.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://lore.kernel.org/r/nycvar.YSQ.7.76.2005021043110.2671@knanqh.ubzr


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