common/xfs: refactor commands to select a particular xfs backing device

Refactor all the places where we try to force new file data allocations to a specific xfs backing device so that we don't end up open-coding the same xfs_io command lines over and over.

Signed-off-by: Darrick J. Wong djwong@kernel.org Reviewed-by: Christoph Hellwig hch@lst.de Signed-off-by: Eryu Guan guaneryu@gmail.com