nvmet: implement valid sqhd values in completions

To support sqhd, for initiators that are following the spec and
paying attention to sqhd vs their sqtail values:

- add sqhd to struct nvmet_sq
- initialize sqhd to 0 in nvmet_sq_setup
- rather than propagate the 0's-based qsize value from the connect message
  which requires a +1 in every sqhd update, and as nothing else references
  it, convert to 1's-based value in nvmt_sq/cq_setup() calls.
- validate connect message sqsize being non-zero per spec.
- updated assign sqhd for every completion that goes back.

Also remove handling the NULL sq case in __nvmet_req_complete, as it can't
happen with the current code.

Signed-off-by: James Smart <james.smart@broadcom.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/nvme/target/fabrics-cmd.c

@@ -109,9 +109,14 @@ static u16 nvmet_install_queue(struct nvmet_ctrl *ctrl, struct nvmet_req *req)
 		pr_warn("queue already connected!\n");
 		return NVME_SC_CONNECT_CTRL_BUSY | NVME_SC_DNR;
 	}
+	if (!sqsize) {
+		pr_warn("queue size zero!\n");
+		return NVME_SC_CONNECT_INVALID_PARAM | NVME_SC_DNR;
+	}
 
-	nvmet_cq_setup(ctrl, req->cq, qid, sqsize);
-	nvmet_sq_setup(ctrl, req->sq, qid, sqsize);
+	/* note: convert queue size from 0's-based value to 1's-based value */
+	nvmet_cq_setup(ctrl, req->cq, qid, sqsize + 1);
+	nvmet_sq_setup(ctrl, req->sq, qid, sqsize + 1);
 	return 0;
 }