LU-1438 quota: fix race in quota_chk_acq_common()

LU-1438 quota: fix race in quota_chk_acq_common()

quota_check_common() & qctxt_adjust_qunit() uses different way
to check if quota is enforced on certain ID, which could result
in infinite loop in quota_chk_acq_common() when the QB/QI_SET
flag is cleared just after checking.

This patch used a non-instrusive way to fix this rare race.

Signed-off-by: Niu Yawei <niu@whamcloud.com>
Change-Id: I7212e9fc85e98a40e36d2773c02f838ca68339bb
Reviewed-on: http://review.whamcloud.com/2927
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Johann Lombardi <johann@whamcloud.com>