dm vdo block-map: remove extra vdo arg from initialize_block_map_zone

The block_map is passed to initialize_block_map_zone, but the
block_map's vdo member is already initialized with the same vdo
instance, so just use it.

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