[XFS] Fixing the error caused by the conflict between DIO Write's

conversion and concurrent truncate operations. Use vn_iowait to wait for
the completion of any pending DIOs. Since the truncate requires exclusive
IOLOCK, so this blocks any further DIO operations since DIO write also
needs exclusive IOBLOCK. This serves as a barrier and prevent any
potential starvation.

SGI-PV: 947420
SGI-Modid: xfs-linux-melb:xfs-kern:208088a

Signed-off-by: Yingping Lu <yingping@sgi.com>
Signed-off-by: Nathan Scott <nathans@sgi.com>