staging/lustre/ptlrpc: false alarm in AT network latency measuring

If early reply of client RPC is lost and client RPC is expired and
resent, server will drop the resent RPC because it's already in
processing, server may also send reply or early reply to client,
which can still match reply buffer of the original request.
In this case, client is measuring time from resent time, but server
is reporting service time of original RPC, which is longer than
the time measured by client.

Signed-off-by: Liang Zhen <liang.zhen@intel.com>
Reviewed-on: http://review.whamcloud.com/12855
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5545


Reviewed-by: Li Wei <wei.g.li@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>