LU-2910 clio: restore iov when restart io

LU-2910 clio: restore iov when restart io

The iovector needs be restored on restarted io. This patch also
added some LASSERT and debug message to make it easier to debug
this kind of problem later.

Test-Parameters: testlist=racer
Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I5d737cfa083ae3b3d9f040a2dc36d6b8693b548b
Reviewed-on: http://review.whamcloud.com/5652
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>