LU-3627 quota: Fix incorrect NULL return value of qsd_init()

LU-3627 quota: Fix incorrect NULL return value of qsd_init()

qsd_init() might return NULL in some cases, which is not correct and
may make kernel crash. This patch fixes this problem.

Signed-off-by: Li Xi <lixi@ddn.com>
Change-Id: I604077f2f9143e500badb5148a6f0d9a246e375f
Reviewed-on: http://review.whamcloud.com/7100
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>