tty: fix a little bug in scrup, vt.c

The code uses vc->vc_cols instead of vc->vc_size_row by mistake, it
will cause half of the region which is going to clear remain
uncleared.
The issue happens in background consoles, so it's hard to observe.

Frank Pan

Signed-off-by: Frank Pan <frankpzh@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>