sync fixes

[Christoph Hellwig]

Sending out the intro mail again as it appears to have been eaten
alive by the list manager.

On Sun, Dec 18, 2011 at 10:49:36AM -0500, Christoph Hellwig wrote:
> These two small fixes fix issues with peridic metadata writeback and
> sync not beeing able write file size updates to disk in some cases.
> 
> The executive summary is:  the writeback code slowly decided that the
> XFS behaviour in corner cases equals to livelocking, and XFS made some
> of this behaviour much more likely since the introduction of the
> delaylog mode.
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