LU-2910 clio: skip iov update when tot_nrsegs is zero

LU-2910 clio: skip iov update when tot_nrsegs is zero

When tot_nrsegs is zero, we should skip the iov update too.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I52d4d52a65802f3967dd6a96ad46ec40fd4ef355
Reviewed-on: http://review.whamcloud.com/5747
Tested-by: Hudson
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>