[XFS] Flush the block device before closing it on unmount.

SGI-PV: 965630
SGI-Modid: xfs-linux-melb:xfs-kern:28774a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Tim Shimmin <tes@sgi.com>