block: allocate the zone bitmaps lazily

Allocate the conventional zone bitmap and the sequential zone locking
bitmap only when we find a zone of the respective type.  This avoids
wasting memory on the conventional zone bitmap for devices that only
have sequential zones, and will also prepare for other future changes.

Reviewed-by: Javier González <javier@javigon.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>