Calculate byte set size for data sample:
- calculate how many unique bytes have been in the sample
- for all bytes count > 0, check if we're still in the low count range
  (~25%), such data are easily compressible, otherwise furhter analysis
  is needed
Signed-off-by: Timofey Titovets <nefelim4ag@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update comments ]
Signed-off-by: David Sterba <dsterba@suse.com>