scsi: target: sbp: Replace use of system_unbound_wq with system_dfl_wq

Currently if a user enqueue a work item using schedule_delayed_work()
the used wq is "system_wq" (per-cpu wq) while queue_delayed_work() use
WORK_CPU_UNBOUND (used when a cpu is not specified). The same applies to
schedule_work() that is using system_wq and queue_work(), that makes use
again of WORK_CPU_UNBOUND.

This lack of consistentcy cannot be addressed without refactoring the
API.

This patch continues the effort to refactor worqueue APIs, which has
begun with the change introducing new workqueues and a new
alloc_workqueue flag:

commit 128ea9f6ccfb ("workqueue: Add system_percpu_wq and system_dfl_wq")
commit 930c2ea566af ("workqueue: Add new WQ_PERCPU flag")

system_dfl_wq should be the default workqueue so as not to enforce
locality constraints for random work whenever it's not required.

The old system_unbound_wq will be kept for a few release cycles.

Suggested-by: Tejun Heo <[e-mail]>
Signed-off-by: Marco Crivellari <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/target/sbp/sbp_target.c

···
 		pr_debug("tgt_agent ORB_POINTER write: 0x%llx\n",
 				agent->orb_pointer);
 
-		queue_work(system_unbound_wq, &agent->work);
+		queue_work(system_dfl_wq, &agent->work);
 
 		return RCODE_COMPLETE;
 
···
 
 		pr_debug("tgt_agent DOORBELL\n");
 
-		queue_work(system_unbound_wq, &agent->work);
+		queue_work(system_dfl_wq, &agent->work);
 
 		return RCODE_COMPLETE;
 
···
 
 		if (tgt_agent_check_active(agent) && !doorbell) {
 			INIT_WORK(&req->work, tgt_agent_process_work);
-			queue_work(system_unbound_wq, &req->work);
+			queue_work(system_dfl_wq, &req->work);
 		} else {
 			/* don't process this request, just check next_ORB */
 			sbp_free_request(req);
···
 		agent->orb_offset = sbp2_pointer_to_addr(ptr);
 		agent->request = req;
 
-		queue_work(system_unbound_wq, &agent->work);
+		queue_work(system_dfl_wq, &agent->work);
 		rcode = RCODE_COMPLETE;
 	} else if (tcode == TCODE_READ_BLOCK_REQUEST) {
 		addr_to_sbp2_pointer(agent->orb_offset, ptr);