btrfs: zoned: show statistics about zoned filesystems in mountstats

btrfs: zoned: show statistics about zoned filesystems in mountstats

Add statistics output to /proc/<pid>/mountstats for zoned BTRFS, similar
to the zoned statistics from XFS in mountstats.

The output for /proc/<pid>/mountstats on an example filesystem will be as
follows:

  device /dev/vda mounted on /mnt with fstype btrfs
    zoned statistics:
          active block-groups: 7
            reclaimable: 0
            unused: 5
            need reclaim: false
          data relocation block-group: 1342177280
          active zones:
            start: 1073741824, wp: 268419072 used: 0, reserved: 268419072, unusable: 0
            start: 1342177280, wp: 0 used: 0, reserved: 0, unusable: 0
            start: 1610612736, wp: 49152 used: 16384, reserved: 16384, unusable: 16384
            start: 1879048192, wp: 950272 used: 131072, reserved: 622592, unusable: 196608
            start: 2147483648, wp: 212238336 used: 0, reserved: 212238336, unusable: 0
            start: 2415919104, wp: 0 used: 0, reserved: 0, unusable: 0
            start: 2684354560, wp: 0 used: 0, reserved: 0, unusable: 0

Reviewed-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>