[Bug-tar] Improve sparse file scan through extent_scan module

[Jeff liu]

Dear developer,

As the fiemap exent-scan module has been added to coreutils for about half 
year, and looks it works stable in the past few releases.

Is it possible to make use of it to improve the current tar sparse file scan 
procedure?
I'd like to work a patch out if you guys like it.


Thanks,
-Jeff