Directly accessing the Nodes does not work properly,
as there is no way to attach a newly LOOKUPped Node
to the tree. This means Path() does not work.
Use an actual mount instead and walk the tree.
If we encounter a 128KB block of zeros, try to skip to the next
data section by calling File.SeekData().
This fixes xfstests generic/285, which creates a 17TB sparse file,
and runs fsck afterwards. Without this optimization, fsck would
take ages.
Most corruption cases except xattr should be covered.
With test filesystem.
The output is still pretty ugly. xattr support will
be added in the next commits.
