xfs: abstract out dev_t conversions

And move them to xfs_linux.h so that xfsprogs can stub them out more
easily.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>