xfs: use WRITE_SYNC_PLUG for synchronous writeout

The VM and I/O schedulers now expect us to use WRITE_SYNC_PLUG for
synchronous writeout.  Right now I can't see any changes in performance
numbers with this, but we're getting some beating for not using it,
and the knowledge definitely could help the block code to make better
decisions.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>
Signed-off-by: Alex Elder <aelder@sgi.com>