block: replace seq_zones_bitmap with conv_zones_bitmap

Invert the meaning of seq_zones_bitmap by keeping a bitmap of
conventional zones.  This allows not having a bitmap for devices
that do not have conventional zones.

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