LU-4249 quota: race in qsd_upd_thread()

LU-4249 quota: race in qsd_upd_thread()

qsd_upd_thread() uses list_for_each_entry_safe() to process
list items one by one, however, it has to drop lock while
processing each item, that'll race with other list processing
thread.

The proper way is to check list head each time when it acquired
lock.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I55dfe3f65cc214c5d0fd8a9afdda795af94bbdce
Reviewed-on: http://review.whamcloud.com/10988
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Jenkins
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>