btrfs: Return number of compressed extents directly in compress_file_range

compress_file_range returns a void, yet uses a function parameter as a
return value. Make that more idiomatic by simply returning the number
of compressed extents directly. Also track such extents in more aptly
named variables. No functional changes.

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