block: Introduce BLKGETNRZONES ioctl

Get a zoned block device total number of zones. The device can be a
partition of the whole device. The number of zones is always 0 for
regular block devices.

Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>