scsi: core: Introduce .queue_reserved_command()

scsi: core: Introduce .queue_reserved_command()

Reserved commands will be used by SCSI LLDs for submitting internal
commands. Since the SCSI host, target and device limits do not apply to
the reserved command use cases, bypass the SCSI host limit checks for
reserved commands. Introduce the .queue_reserved_command() callback for
reserved commands. Additionally, do not activate the SCSI error handler
if a reserved command fails such that reserved commands can be submitted
from inside the SCSI error handler.

[ bvanassche: modified patch title and patch description. Renamed
  .reserved_queuecommand() into .queue_reserved_command(). Changed
  the second argument of __blk_mq_end_request() from 0 into error
  code in the completion path if cmd->result != 0. Rewrote the
  scsi_queue_rq() changes. See also
  https://lore.kernel.org/linux-scsi/1666693096-180008-5-git-send-email-john.garry@huawei.com/ ]

Cc: Hannes Reinecke <hare@suse.de>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://patch.msgid.link/20251031204029.2883185-6-bvanassche@acm.org
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>