lustre/ptlrpc: fix 'data race condition' issues

Fix 'data race condition' defects found by Coverity version
6.5.0:
Data race condition (MISSING_LOCK)
Accessing variable without holding lock. Elsewhere,
this variable is accessed with lock held.

Signed-off-by: Sebastien Buisson <sebastien.buisson@bull.net>
Reviewed-on: http://review.whamcloud.com/6575
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-2744


Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Oeg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>