In the zoned mode there are parts of chunks that become unusable once
they get COWed and the zone must be reclaimed and reset to make the
space usable again. Provide a way to show the total size per block group
type in fi df:
$ btrfs fi df .
Data, single: total=1.00GiB, used=257.51MiB, zone_unusable=238.43MiB
System, single: total=256.00MiB, used=16.00KiB, zone_unusable=224.00KiB
Metadata, single: total=256.00MiB, used=816.00KiB, zone_unusable=8.61MiB
GlobalReserve, single: total=3.25MiB, used=0.00B
This will not be shown on non-zoned filesystems.
Signed-off-by: David Sterba <dsterba@suse.com>