xfs: factor out debug checks from xfs_dialloc and xfs_difree

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: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>
Signed-off-by: Felix Blyakher <felixb@sgi.com>