scsi: core: Support allocating reserved commands

scsi: core: Support allocating reserved commands

Quite some drivers are using management commands internally. These
commands typically use the same tag pool as regular SCSI commands. Tags
for these management commands are set aside before allocating the
block-mq tag bitmap for regular SCSI commands. The block layer already
supports this via the reserved tag mechanism. Add a new field
'nr_reserved_cmds' to the SCSI host template to instruct the block layer
to set aside a tag space for these management commands by using reserved
tags. Exclude reserved commands from .can_queue because .can_queue is
visible in sysfs.

[ bvanassche: modified patch title and patch description. Left out the
  following statements: "if (sht->nr_reserved_cmds)" and also
  "if (sdev->host->nr_reserved_cmds) flags |= BLK_MQ_REQ_RESERVED;". Moved
  nr_reserved_cmds declarations and statements close to the
  corresponding can_queue declarations and statements. See also
  https://lore.kernel.org/linux-scsi/20210503150333.130310-11-hare@suse.de/ ]

Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://patch.msgid.link/20251031204029.2883185-2-bvanassche@acm.org
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>