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.

Change-Id: I9d3794426393f9e0da7932b05acd8f4028a12cb3
Reviewed-on: http://gerrit.openafs.org/2960
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>