vt: selection, use rounddown() for start/endline computation

We have a helper called rounddown for these modulo computations. So use
it.

No functional change intended and "objdump -d" proves that.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200316065911.11024-2-jslaby@suse.cz


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