The previous commit is not enough as a fix.  This may happen:

    - we invalidate stamps after processing them
    - we send request
    - get reply, loose reply-fup
    - send request
    - loose reply, get f-up

So we now invalidate when sending the request. And invalidate t4 alone
as the beautifulness and symmetry of the previous commit is lost
anyways.

Note: there no need to invalidate stamps in e2e mode, because checking
the sequence number to validate RX frames is enough.  But here all
replies match the sequence number, so the problem is not caught and
stamps from different tuples are mixed.

Example beofre this commit, with trimmed stamps (was 1497283863):

   diag-frames-1-wr1: SENT 54 bytes at 863.333173928 (pdelay_req)
   diag-frames-1-wr1: RECV 54 bytes at 863.334158796 (type 3, pdelay_resp)
   diag-frames-1-wr1: Drop received frame
   diag-frames-1-wr1: SENT 54 bytes at 864.479336104 (pdelay_req)
   diag-frames-1-wr1: Drop received frame
   diag-frames-1-wr1: RECV 54 bytes at 864.481095164 (type a, presp_follow_up)

   diag-servo-2-wr1: servo:t3 = 864:479336104:0
   diag-servo-2-wr1: servo:t4 = 863:333174267:586
   diag-servo-2-wr1: servo:t5 = 864:480295312:0
   diag-servo-2-wr1: servo:t6 = 863:334158796:773
   diag-servo-2-wr1: ->mdelay = -2:-292298352:359
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>