I think the right fix is to move fiemap_check_ranges into all the ->fiemap instances (we only have a few actual implementation minus the wrappers around iomap/generic). Then add a version if iomap_fiemap that can pass in maxbytes explicitly for ext4, similar to what we've done with various other generic helpers. The idea of validating input against file systems specific paramaters before we call into the fs is just bound to cause problems.
