staging/hv/osd: don't reimplement ALIGN macro

The ALIGN_DOWN macro was only used in NUM_PAGES_SPANNED.  So make the
latter easier and get rid of the former.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>