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.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
diff --git a/libxfs/xfs_format.h b/libxfs/xfs_format.h
index f169741..e6ca188 100644
--- a/libxfs/xfs_format.h
+++ b/libxfs/xfs_format.h
@@ -1272,6 +1272,9 @@
 #define XFS_DQ_GRACE_MIN		((int64_t)0)
 #define XFS_DQ_GRACE_MAX		((int64_t)U32_MAX)
 
+/* Maximum id value for a quota record */
+#define XFS_DQ_ID_MAX			(U32_MAX)
+
 /*
  * This is the main portion of the on-disk representation of quota information
  * for a user.  We pad this with some more expansion room to construct the on