linvdimm, pmem: Preserve read-only setting for pmem devices

The pmem driver does not honor a forced read-only setting for very long:
$ blockdev --setro /dev/pmem0
$ blockdev --getro /dev/pmem0
1

followed by various commands like these:
$ blockdev --rereadpt /dev/pmem0
or
$ mkfs.ext4 /dev/pmem0

results in this in the kernel serial log:
nd_pmem namespace0.0: region0 read-write, marking pmem0 read-write

with the read-only setting lost:
$ blockdev --getro /dev/pmem0
0

That's from bus.c nvdimm_revalidate_disk(), which always applies the
setting from nd_region (which is initially based on the ACPI NFIT
NVDIMM state flags not_armed bit).

In contrast, commit 20bd1d026aac ("scsi: sd: Keep disk read-only when
re-reading partition") fixed this issue for SCSI devices to preserve
the previous setting if it was set to read-only.

This patch modifies bus.c to preserve any previous read-only setting.
It also eliminates the kernel serial log print except for cases where
read-write is changed to read-only, so it doesn't print read-only to
read-only non-changes.

Cc: <[e-mail]>
Fixes: 581388209405 ("libnvdimm, nfit: handle unarmed dimms, mark namespaces read-only")
Signed-off-by: Robert Elliott <[e-mail]>
Signed-off-by: Dan Williams <[e-mail]>

drivers/nvdimm/bus.c

···
 {
 	struct device *dev = disk_to_dev(disk)->parent;
 	struct nd_region *nd_region = to_nd_region(dev->parent);
-	const char *pol = nd_region->ro ? "only" : "write";
+	int disk_ro = get_disk_ro(disk);
 
-	if (nd_region->ro == get_disk_ro(disk))
+	/*
+	 * Upgrade to read-only if the region is read-only preserve as
+	 * read-only if the disk is already read-only.
+	 */
+	if (disk_ro || nd_region->ro == disk_ro)
 		return 0;
 
-	dev_info(dev, "%s read-%s, marking %s read-%s\n",
-			dev_name(&nd_region->dev), pol, disk->disk_name, pol);
-	set_disk_ro(disk, nd_region->ro);
+	dev_info(dev, "%s read-only, marking %s read-only\n",
+			dev_name(&nd_region->dev), disk->disk_name);
+	set_disk_ro(disk, 1);
 
 	return 0;