btrfs: zoned: show statistics for zoned filesystems

btrfs: zoned: show statistics for zoned filesystems

Provide statistics for zoned filesystems. These statistics include, the
number of active block-groups, how many of them are reclaimable or unused,
if the filesystem needs to be reclaimed, the currently assigned relocation
and treelog block-groups if they're present and a list of active zones.

Example:
  active block-groups: 4
    reclaimable: 0
    unused: 2
    need reclaim: false
  data relocation block-group: 4294967296
  active zones:
    start: 1610612736, wp: 344064 used: 16384, reserved: 0, unusable: 327680
    start: 1879048192, wp: 34963456 used: 131072, reserved: 0, unusable: 34832384
    start: 4026531840, wp: 0 used: 0, reserved: 0, unusable: 0
    start: 4294967296, wp: 0 used: 0, reserved: 0, unusable: 0

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>