btrfs: zoned: fix incorrect ASSERT in btrfs_zoned_reserve_data_reloc_bg()

When moving a block-group to the dedicated data relocation space-info in
btrfs_zoned_reserve_data_reloc_bg() it is asserted that the newly
created block group for data relocation does not contain any
zone_unusable bytes.

But on disks with zone_capacity < zone_size, the difference between
zone_size and zone_capacity is accounted as zone_unusable.

Instead of asserting that the block-group does not contain any
zone_unusable bytes, remove them from the block-groups total size.

Reported-by: Yi Zhang <yi.zhang@redhat.com>
Link: https://lore.kernel.org/linux-block/CAHj4cs8-cS2E+-xQ-d2Bj6vMJZ+CwT_cbdWBTju4BV35LsvEYw@mail.gmail.com/
Fixes: daa0fde ("btrfs: zoned: fix data relocation block group reservation")
Reviewed-by: Naohiro Aota <naohiro.aota@wdc.com>
Tested-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: morbidrsa

fs/btrfs/zoned.c

@@ -2582,9 +2582,9 @@ void btrfs_zoned_reserve_data_reloc_bg(struct btrfs_fs_info *fs_info)
 			spin_lock(&space_info->lock);
 			space_info->total_bytes -= bg->length;
 			space_info->disk_total -= bg->length * factor;
+			space_info->disk_total -= bg->zone_unusable;
 			/* There is no allocation ever happened. */
 			ASSERT(bg->used == 0);
-			ASSERT(bg->zone_unusable == 0);
 			/* No super block in a block group on the zoned setup. */
 			ASSERT(bg->bytes_super == 0);
 			spin_unlock(&space_info->lock);