Factor out a common helper from repeated debug checks in xfs_dialloc and
xfs_difree.

[hch: split out from Dave's dynamic allocation policy patches]
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NAlex Elder <aelder@sgi.com>
Signed-off-by: NFelix Blyakher <felixb@sgi.com>