to save on env allocation.

Benchmarks made by Shuichi Ihara demonstrated 13% improvement
for small I/Os: 564k vs 639k IOPS. the details can be found
LU-11164.

Change-Id: I797e3d7e19ef408993004a2b872842d655240525
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/32832


Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Patrick Farrell <paf@cray.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>