libnvdimm: allow legacy (e820) pmem region to clear bad blocks

Bad blocks can be injected via /sys/block/pmemN/badblocks. In a situation
where legacy pmem is being used or a pmem region created by using memmap
kernel parameter, the injected bad blocks are not cleared due to
nvdimm_clear_poison() failing from lack of ndctl function pointer. In
this case we need to just return as handled and allow the bad blocks to
be cleared rather than fail.
Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/nvdimm/bus.c

@@ -185,8 +185,12 @@ long nvdimm_clear_poison(struct device *dev, phys_addr_t phys,
 		return -ENXIO;
 
 	nd_desc = nvdimm_bus->nd_desc;
+	/*
+	 * if ndctl does not exist, it's PMEM_LEGACY and
+	 * we want to just pretend everything is handled.
+	 */
 	if (!nd_desc->ndctl)
-		return -ENXIO;
+		return len;
 
 	memset(&ars_cap, 0, sizeof(ars_cap));
 	ars_cap.address = phys;