MRP-1180 mdt: fix error handling in mdt_quotactl_handle()

MDT handlers should return errors with SERIOUS
flag or allocate reply buffer.
Otherwise assertion fails:
niobuf.c:428:ptlrpc_send_reply()) ASSERTION( rs != ((void *)0) ) failed:

Change-Id: Iff83dbb968710e54c9c9bd941af399f129c26ba5
Signed-off-by: Andriy Skulysh <Andriy_Skulysh@xyratex.com>
Reviewed-on: http://morpheus.xyus.xyratex.com:8443/gerrit/566
Tested-by: Jenkins
Reviewed-by: Alexander Boyko <Alexander_Boyko@xyratex.com>
Tested-by: Elena Gryaznova <Elena_Gryaznova@xyratex.com>
Reviewed-by: Alexey Lyashkov <Alexey_Lyashkov@xyratex.com>
Reviewed-by: Vitaly Fertman <Vitaly_Fertman@xyratex.com>
Reviewed-on: http://morpheus.xyus.xyratex.com:8443/gerrit/643
Reviewed-by: Denis Kondratenko <Denis_Kondratenko@xyratex.com>