rx: Simplify round trip time calculation

rx: Simplify round trip time calculation

Move the logic for deciding whether to compute RTT out of PeerNetStats
and into the callers. This means that we can share decisions about
whether a packet is ACK'd or not, and avoid uneccessary multiple tests
and function calls.

This change also stops us from computing RTT times for packets outside
of the set of explicit ACKs that we have received. This means that we
no longer compute RTTs for packets that are on the transmit queue, but
not yet on the wire.

Reviewed-on: http://gerrit.openafs.org/2960
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit f7799a35c5942ba4feda3bc8c848cdaa8e0b7b76)
Change-Id: Idf4ba83ba187fd97caa85d0d042662c3b12efa02
Reviewed-on: http://gerrit.openafs.org/3113