nvmet: fix mar and mor off-by-one errors

Maximum Active Resources (MAR) and Maximum Open Resources (MOR) are 0's
based vales where a value of 0xffffffff indicates that there is no limit.

Decrement the values that are returned by bdev_max_open_zones and
bdev_max_active_zones as the block layer helpers are not 0's based.
A 0 returned by the block layer helpers indicates no limit, thus convert
it to 0xffffffff (U32_MAX).

Fixes: aaf2e04 ("nvmet: add ZBD over ZNS backend support")
Suggested-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Dennis Maisenbacher <dennis.maisenbacher@wdc.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/target/zns.c

@@ -100,6 +100,7 @@ void nvmet_execute_identify_cns_cs_ns(struct nvmet_req *req)
 	struct nvme_id_ns_zns *id_zns;
 	u64 zsze;
 	u16 status;
+	u32 mar, mor;
 
 	if (le32_to_cpu(req->cmd->identify.nsid) == NVME_NSID_ALL) {
 		req->error_loc = offsetof(struct nvme_identify, nsid);
@@ -130,8 +131,20 @@ void nvmet_execute_identify_cns_cs_ns(struct nvmet_req *req)
 	zsze = (bdev_zone_sectors(req->ns->bdev) << 9) >>
 					req->ns->blksize_shift;
 	id_zns->lbafe[0].zsze = cpu_to_le64(zsze);
-	id_zns->mor = cpu_to_le32(bdev_max_open_zones(req->ns->bdev));
-	id_zns->mar = cpu_to_le32(bdev_max_active_zones(req->ns->bdev));
+
+	mor = bdev_max_open_zones(req->ns->bdev);
+	if (!mor)
+		mor = U32_MAX;
+	else
+		mor--;
+	id_zns->mor = cpu_to_le32(mor);
+
+	mar = bdev_max_active_zones(req->ns->bdev);
+	if (!mar)
+		mar = U32_MAX;
+	else
+		mar--;
+	id_zns->mar = cpu_to_le32(mar);
 
 done:
 	status = nvmet_copy_to_sgl(req, 0, id_zns, sizeof(*id_zns));