LU-935 quota: break early when b/i_unit_sz exceeded upper limit

While expanding b/i_unit_sz in dquot_create_oqaq(), we'd break the loop
early when the b/i_unit_sz exceeded upper limit, otherwise, qaq_b/iunit_sz
could be overflow and result in endless loop.

Signed-off-by: Niu Yawei <niu@whamcloud.com>
Change-Id: I0bf069e9259627426d7a87ec42844eaed7a733b4
Reviewed-on: http://review.whamcloud.com/1890
Tested-by: Hudson
Reviewed-by: Johann Lombardi <johann@whamcloud.com>
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>