LU-2211 quota: cap how long a thread can wait for quota

LU-2211 quota: cap how long a thread can wait for quota

Change qsd_op_begin() path to wait for quota space for less than
obd_timeout / 2.
This patch also abandons the qsd_ops enum in favor of a more generic
qsd_adjust() implementation which will always do the same processing
even if adjustment is delayed because of a quota request in flight.

Signed-off-by: Johann Lombard <johann.lombardi@intel.com>
Signed-off-by: Niu Yawei <niu@whamcloud.com>
Change-Id: I5faf637c5330ca7f503c292e0e28edb84458ee89
Reviewed-on: http://review.whamcloud.com/4314
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>