staging/lustre/ptlrpc: hold rq_lock when modify rq_flags

In after_reply(), take the rq_lock for changing the rq_resend.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-on: http://review.whamcloud.com/11957
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5633


Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>