xfs: improve dquot iteration for scrub

Upon a closer inspection of the quota record scrubber, I noticed that
dqiterate wasn't actually walking all possible dquots for the mapped
blocks in the quota file.  This is due to xfs_qm_dqget_next skipping all
XFS_IS_DQUOT_UNINITIALIZED dquots.

For a fsck program, we really want to look at all the dquots, even if
all counters and limits in the dquot record are zero.  Rewrite the
implementation to do this, as well as switching to an iterator paradigm
to reduce the number of indirect calls.

This enables removal of the old broken dqiterate code from xfs_dquot.c.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>