block: introduce zone capacity helper

{bdev,disk}_zone_capacity() takes block_device or gendisk and sector position
and returns the zone capacity of the corresponding zone.

With that, move disk_nr_zones() and blk_zone_plug_bio() to consolidate them in
the same #ifdef block.

Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: David Sterba <dsterba@suse.com>