vt: keyboard, union perm checks in vt_do_kdgkb_ioctl

Do the permission check on a single place. That is where perm is
really checked.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20201029113222.32640-14-jslaby@suse.cz


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