scsi: always assign block layer tags if enabled

Allow a driver to ask for block layer tags by setting .use_blk_tags in the
host template, in which case it will always see a valid value in
request->tag, similar to the behavior when using blk-mq.  This means even
SCSI "untagged" commands will now have a tag, which is especially useful
when using a host-wide tag map.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mike Christie <michaelc@cs.wisc.edu>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>

diff --cc Documentation/scsi/scsi_mid_low_api.txt
Simple merge

diff --cc drivers/message/fusion/mptsas.c
Simple merge

diff --cc drivers/scsi/53c700.c
Simple merge

diff --cc drivers/scsi/aic7xxx/aic79xx_osm.c
Simple merge

diff --cc drivers/scsi/aic7xxx/aic7xxx_osm.c
Simple merge

diff --cc drivers/scsi/aic94xx/aic94xx_init.c
Simple merge

diff --cc drivers/scsi/bfa/bfad_im.c
Simple merge

diff --cc drivers/scsi/bnx2fc/bnx2fc_fcoe.c
Simple merge

diff --cc drivers/scsi/csiostor/csio_scsi.c
Simple merge

diff --cc drivers/scsi/esas2r/esas2r_main.c
Simple merge

diff --cc drivers/scsi/esp_scsi.c
Simple merge

diff --cc drivers/scsi/fcoe/fcoe.c
Simple merge

diff --cc drivers/scsi/fnic/fnic_main.c
Simple merge

diff --cc drivers/scsi/ibmvscsi/ibmvfc.c
Simple merge

diff --cc drivers/scsi/ipr.c
Simple merge

diff --cc drivers/scsi/isci/init.c
Simple merge

diff --cc drivers/scsi/libfc/fc_fcp.c
Simple merge

diff --cc drivers/scsi/libsas/sas_scsi_host.c
Simple merge

diff --cc drivers/scsi/lpfc/lpfc_scsi.c
Simple merge

diff --cc drivers/scsi/mvsas/mv_init.c
Simple merge

diff --cc drivers/scsi/pm8001/pm8001_init.c
Simple merge

diff --cc drivers/scsi/pmcraid.c
Simple merge

diff --cc drivers/scsi/qla2xxx/qla_os.c
Simple merge

diff --cc drivers/scsi/qla4xxx/ql4_os.c
Simple merge

diff --cc drivers/scsi/scsi.c
Simple merge

diff --cc drivers/scsi/scsi_scan.c
Simple merge

diff --cc drivers/scsi/stex.c
Simple merge

diff --cc drivers/scsi/tmscsim.c
Simple merge

diff --cc drivers/scsi/ufs/ufshcd.c
Simple merge

diff --cc drivers/target/loopback/tcm_loop.c
Simple merge

diff --cc drivers/usb/storage/uas.c
Simple merge

diff --cc include/scsi/scsi_host.h
Simple merge

diff --cc include/scsi/scsi_tcq.h
Simple merge