SEEK_DATA/SEEK_HOLE support

[Christoph Hellwig]

On Mon, Oct 03, 2011 at 12:06:37AM +0800, Jeff Liu wrote:
> IMHO, to avoid data loss in some user application like cp(1), for
> unwritten extents, we always need to check the pages status.  Just as
> you mentioned above, return the map offset if pages are dirty for
> SEEK_DATA, or a hole found.

I'd suggest to first implement the simple versions I schemed below,
which would treat unwritten extents as data.  That is sub-optimal,
but a) safe and b) easy to implement.  The second step would be to
add probing for unwritten extents, which is even something we could
do as a common helper routine shared by filesystems.

And the most important thing is of course adding QA for it.  Josef
already wrote an xfstests case that needs to be resurrected, compared
against the latest Posix draft and if nessecary updated.