block: new zoned loop block device driver

The zoned loop block device driver allows a user to create emulated
zoned block devices using one regular file per zone as backing storage.
Compared to null_blk or scsi_debug, it has the advantage of allowing
emulating large zoned devices without requiring the same amount of
memory as the capacity of the emulated device. Furthermore, zoned
devices emulated with this driver can be re-started after a host reboot
without any loss of the state of the device zones, which is something
that null_blk and scsi_debug do not support.

This initial implementation is simple and does not support zone resource
limits. That is, a zoned loop block device limits for the maximum number
of open zones and maximum number of active zones is always 0.

This driver can be either compiled in-kernel or as a module, named
"zloop". Compilation of this driver depends on the block layer support
for zoned block device (CONFIG_BLK_DEV_ZONED must be set).

Using the zloop driver to create and delete zoned block devices is
done by writing commands to the zoned loop control character device file
(/dev/zloop-control). Creating a device is done with:

  $ echo "add [options]" > /dev/zloop-control

The options available for the "add" operation cat be listed by reading
the zloop-control device file:

  $ cat /dev/zloop-control
  add id=%d,capacity_mb=%u,zone_size_mb=%u,zone_capacity_mb=%u,conv_zones=%u,base_dir=%s,nr_queues=%u,queue_depth=%u
  remove id=%d

The options available allow controlling the zoned device total
capacity, zone size, zone capactity of sequential zones, total number
of conventional zones, base directory for the zones backing file, number
of I/O queues and the maximum queue depth of I/O queues.

Deleting a device is done using the "remove" command:

  $ echo "remove id=0" > /dev/zloop-control

This implementation passes various tests using zonefs and fio (t/zbd
tests) and provides a state machine for zone conditions that is
compliant with the T10 ZBC and NVMe ZNS specifications.

Co-developed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20250407075222.170336-2-dlemoal@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

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