LU-2099 osd: clear iobuf up on I/O completion

LU-2099 osd: clear iobuf up on I/O completion

it was a bad idea to clear iobuf on buffer release because the
same buffers can be used few times in case of transient ENOSPC.

Change-Id: Ic1d2ddaa0794f2f6f7d1ba71620e06b4ea7a99d2
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-on: http://review.whamcloud.com/10305
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Tested-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>