The check is pretty simple, just two checks:
- Tree owner check
  Skinny block group item should only exist in block group tree.

- Used num bytes (key->offset)
  To avoid possible later chunk size change, here we use super large
  value (64T) as threshold to reduce false alert.

- Duplicated skinny block group items
  There shouldn't be duplicated items for the same block group.

Signed-off-by: Qu Wenruo <wqu@suse.com>