scsi: target: xen-scsiback: Convert to new submission API

target_submit_cmd_map_sgls() is being removed, so convert xen to the new
submission API. This has it use target_init_cmd(), target_submit_prep(), or
target_submit() because we need to have LIO core map sgls which is now done
in target_submit_prep(). target_init_cmd() will never fail for xen because
it does its own sync during session shutdown, so we can remove that code.

Note: xen never calls target_stop_session() so target_submit_cmd_map_sgls()
never failed (in the new API target_init_cmd() handles
target_stop_session() being called when cmds are being submitted). If it
were to have used target_stop_session() and got an error, we would have hit
a refcount bug like xen and usb, because it does:

    if (rc < 0) {
            transport_send_check_condition_and_sense(se_cmd,
                            TCM_LOGICAL_UNIT_COMMUNICATION_FAILURE, 0);
            transport_generic_free_cmd(se_cmd, 0);
    }

transport_send_check_condition_and_sense() calls queue_status which calls
scsiback_cmd_done->target_put_sess_cmd. We do an extra
transport_generic_free_cmd() call above which would have dropped the
refcount to -1 and the refcount code would spit out errors.

Link: https://lore.kernel.org/r/20210227170006.5077-13-michael.christie@oracle.com
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/xen/xen-scsiback.c

@@ -360,21 +360,18 @@ static void scsiback_cmd_exec(struct vscsibk_pend *pending_req)
 {
 	struct se_cmd *se_cmd = &pending_req->se_cmd;
 	struct se_session *sess = pending_req->v2p->tpg->tpg_nexus->tvn_se_sess;
-	int rc;
 
 	scsiback_get(pending_req->info);
 	se_cmd->tag = pending_req->rqid;
-	rc = target_submit_cmd_map_sgls(se_cmd, sess, pending_req->cmnd,
-			pending_req->sense_buffer, pending_req->v2p->lun,
-			pending_req->data_len, 0,
-			pending_req->sc_data_direction, TARGET_SCF_ACK_KREF,
-			pending_req->sgl, pending_req->n_sg,
-			NULL, 0, NULL, 0);
-	if (rc < 0) {
-		transport_send_check_condition_and_sense(se_cmd,
-				TCM_LOGICAL_UNIT_COMMUNICATION_FAILURE, 0);
-		transport_generic_free_cmd(se_cmd, 0);
-	}
+	target_init_cmd(se_cmd, sess, pending_req->sense_buffer,
+			pending_req->v2p->lun, pending_req->data_len, 0,
+			pending_req->sc_data_direction, TARGET_SCF_ACK_KREF);
+
+	if (target_submit_prep(se_cmd, pending_req->cmnd, pending_req->sgl,
+			       pending_req->n_sg, NULL, 0, NULL, 0))
+		return;
+
+	target_submit(se_cmd);
 }
 
 static int scsiback_gnttab_data_map_batch(struct gnttab_map_grant_ref *map,