btrfs: zoned: use regular super block location on zone emulation

A zoned filesystem currently has a superblock at the beginning of the
superblock logging zones if the zones are conventional. This difference
in superblock position causes a chicken-and-egg problem for filesystems
with emulated zones. Since the device is a regular (non-zoned) device,
we cannot know if the filesystem is regular or zoned while reading the
superblock. But, to load the superblock, we need to see if it is
emulated zoned or not.

Place the superblocks at the same location as they are on regular
filesystem on regular devices to solve the problem. It is possible
because it's ensured that all the superblock locations are at an
(emulated) conventional zone on regular devices.
Reviewed-by: NAnand Jain <anand.jain@oracle.com>
Reviewed-by: NJosef Bacik <josef@toxicpanda.com>
Signed-off-by: NNaohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/zoned.c

@@ -552,7 +552,13 @@ int btrfs_sb_log_location(struct btrfs_device *device, int mirror, int rw,
 	struct btrfs_zoned_device_info *zinfo = device->zone_info;
 	u32 zone_num;
 
-	if (!zinfo) {
+	/*
+	 * For a zoned filesystem on a non-zoned block device, use the same
+	 * super block locations as regular filesystem. Doing so, the super
+	 * block can always be retrieved and the zoned flag of the volume
+	 * detected from the super block information.
+	 */
+	if (!bdev_is_zoned(device->bdev)) {
 		*bytenr_ret = btrfs_sb_offset(mirror);
 		return 0;
 	}