scsi: libsas: Add sas_task_find_rq()

blk-mq already provides a unique tag per request. Some libsas LLDDs - like
hisi_sas - already use this tag as the unique per-I/O HW tag.

Add a common function to provide the request associated with a sas_task for
all libsas LLDDs.

Signed-off-by: John Garry <john.garry@huawei.com>
Link: https://lore.kernel.org/r/1666091763-11023-2-git-send-email-john.garry@huawei.com
Reviewed-by: Jack Wang <jinpu.wang@ionos.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>