dm vdo: replace max_discard_sectors with max_hw_discard_sectors

Commit 4f563a64 ("block: add a max_user_discard_sectors queue
limit") changed block core to set max_discard_sectors to:
min(lim->max_hw_discard_sectors, lim->max_user_discard_sectors)

Commit 825d8bbd ("dm: always manage discard support in terms
of max_hw_discard_sectors") fixed most dm targetss to deal with
this, by replacing max_discard_sectors with max_hw_discard_sectors.
Unfortunately, dm-vdo did not get fixed at that time.

Fixes: 825d8bbd ("dm: always manage discard support in terms of max_hw_discard_sectors")
Signed-off-by: Bruce Johnston <bjohnsto@redhat.com>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>