dm vdo block-map: avoid extra dereferences to access vdo object

The vdo_page_cache's 'vdo' is the same as the block_map's vdo
instance, so use that to save 2 extra dereferences.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Matthew Sakai <msakai@redhat.com>