In response to the discussion of the xfstests mailing list [1],
I looked at the Lseek implementation, which was naive and
did not handle all cases correctly.
The new implementation aligns the returned values to 4096 bytes
as most callers expect.
A lot of tests are added to verify that we handle all
cases correctly now.
[1]: https://www.spinics.net/lists/fstests/msg16554.html
738a9e006a