staging/lustre/obdclass: fix a race in recovery

in "class_export_recovery_cleanup", the check of the flag
"exp->exp_req_replay_needed" should be protected by "exp_lock".

Signed-off-by: Hongchao Zhang <hongchao.zhang@intel.com>
Reviewed-on: http://review.whamcloud.com/10849
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5128
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>