LU-1830 quota: fix ldiskfs quota accouning iterator

There were defects in the ldiskfs quota accounting iterator:
- leaf blocks could be re-processed;
- entry index was updated incorrectly, so some entries could be
  skipped;

Fix the defect in osd_diskfs_read(), the 32bit 'size' could be
overflowed.

Signed-off-by: Niu Yawei <niu@whamcloud.com>
Change-Id: Icfd96fdaf38c57bc63a5fcfd1f96c02d0ddeb74d
Reviewed-on: http://review.whamcloud.com/3877
Reviewed-by: Johann Lombardi <johann@whamcloud.com>
Tested-by: Hudson
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>