vt: extract attribute inversion to vc_invert_attr

We continue cleaning up do_con_write. This (hopefully) makes the
inversion code obvious.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200615074910.19267-14-jslaby@suse.cz


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