btrfs: sysfs: add size class stats

Make it possible to see the distribution of size classes for block
groups. Helpful for testing and debugging the allocator w.r.t. to size
classes.

The new stats can be found at the path:

  /sys/fs/btrfs/<FSID>/allocation/<bg-type>/size_class

but they will only be non-zero for bg-type = data.

Signed-off-by: Boris Burkov <boris@bur.io>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>