The request associated with a SCSI command coming from the block layer has
a unique tag, so use that when possible for getting a slot.

Unfortunately we don't support reserved commands in the SCSI midlayer yet.
As such, SMP tasks - as an example - will not have a request associated, so
in the interim continue to manage those tags for that type of sas_task
internally.

We reserve an arbitrary 4 tags for these internal tags. Indeed, we already
decrement MVS_RSVD_SLOTS by 2 for the shost can_queue when flag
MVF_FLAG_SOC is set. This change was made in commit 20b09c29 ("[SCSI]
mvsas: add support for 94xx; layout change; bug fixes"), but what those 2
slots are used for is not obvious.

Also make the tag management functions static, where possible.
Signed-off-by: John Garry <john.garry@huawei.com>
Link: https://lore.kernel.org/r/1666091763-11023-8-git-send-email-john.garry@huawei.comSigned-off-by: Martin K. Petersen <martin.petersen@oracle.com>