Make DataSN counter of solicited Data-Out local.

DataSN for solicited Data-Out is per-R2T. Since we handle whole R2T
in one go, we don't need to store it anywhere, especially in global
per-command structure. This may allow us to handle multiple R2T per
command at once, if we decide, or may be relax locking.

Rename the second use of that field to io_referenced_task_tag.

MFC after: 1 month

(cherry picked from commit 3dd2a7a5ea2f1641c7525f692eed416fa02c28e6)