Support for uncompressed sparse files.

[Pratik Shinde]

Hello Gao,

I have started working on above functionality for erofs.
First thing we need to do is detect sparse files & determine location of
holes in it.

I was thinking of using lseek() with SEEK_HOLE & SEEK_DATA for detecting
holes.
Let me know what you think about the approach OR any other better approach
in your mind.

PS : support for SEEK_HOLE & SEEK_DATA came in 3.4 kernel.