Btrfs: compress_file_range remove dead variable num_bytes

Remove dead assigment of num_bytes.

Also as num_bytes only used in the will_compress block as copy of
total_in just replace that with total_in and drop num_bytes entirely.

Signed-off-by: Timofey Titovets <nefelim4ag@gmail.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>