xfs: refactor dquot iteration

Create a helper function to iterate all the dquots of a given type in
the system, and refactor the dquot scrub to use it.  This will get more
use in the quota repair code.

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