LU-8346 obdclass: protect key_set_version

LU-8346 obdclass: protect key_set_version

In lu_context_refill, the key_set_version should be protected
before comparing it to version stored in the lu_context.

This patch is a supplement of the previous patch
https://review.whamcloud.com/#/c/28405/, which adds protection
for key_set_version from modification in lu_context_refill
and lu_context_key_degister.

Change-Id: I201f56214382a717cfc31ba573e06fec9fbedae4
Signed-off-by: Hongchao Zhang <hongchao.zhang@intel.com>
Reviewed-on: https://review.whamcloud.com/27448
Reviewed-by: Patrick Farrell <paf@cray.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>