Add support for host_tagset.

Also move the reserved command slots to the end of the pool to eliminate an
addition operation for every SCSI request.

This patch was originally authored by Hannes Reinecke here:

Link: https://lore.kernel.org/linux-block/20191126131009.71726-8-hare@suse.de/

But we NAKed this patch because we wanted to fully test multipath
failover operations.
Suggested-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Scott Benesh <scott.benesh@microchip.com>
Reviewed-by: Scott Teel <scott.teel@microchip.com>
Reviewed-by: Mahesh Rajashekhara <Mahesh.Rajashekhara@microchip.com>
Reviewed-by: Mike McGowen <Mike.McGowen@microchip.com>
Reviewed-by: Kevin Barnett <kevin.barnett@microchip.com>
Signed-off-by: Don Brace <don.brace@microchip.com>
Link: https://lore.kernel.org/r/166793529811.322537.3294617845448383948.stgit@brunhildaSigned-off-by: Martin K. Petersen <martin.petersen@oracle.com>