Filesystem allocation bitmap

[Kai Meyer]

Is there a generic filesystem method to retrieve the filesystem's 
allocation bitmap? I'm mostly interested in Ext filesystems, so if 
there's nothing generic, I'm happy with a specific solution for just 
Ext. If the answer is "read 
https://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout and then read 
the bitmaps directly from disk", I think I can deal with that too.

-Kai Meyer