rx: Account for delayed ACKS when computing RTO

rx: Account for delayed ACKS when computing RTO

RX currently only soft ACKs every second packet, therefore a soft ACK
may be delayed by a period of time (currently 100ms, although RX did
expose this as a public variable in earlier versions).

RTT values are computed using only non-delayed ACKs, so the timeout
is a smoothed average of the exact time taken to send and directly
ACK a packet. Therefore, if the peer ends up using a delayed ACK for
the packet, using just the RTT will cause that packet to be timed out.

A while ago, this was dealt with by padding the calculated RTT with an
additional 350ms. This was then removed, and changed to a 350ms minimum
value. When this caused large numbers of spurious resends, the padding
was restored, but with a 20ms default value. As noted above, 20ms is
too low, as we may wait for up to 100ms before sending an ACK.

This patch changes minPeerTimeout so that it does what it says on
the tin - sets a minimum value below which the peer timout may not
fall. It then adds to either this value, or the calculated one, 200ms
of padding. This makes our padding identical to TCPs, and allows some
future leway as to the softAckDelay value.

Change-Id: I48ab28e03bb7c0a49fe21c21f83adbb02b7665f2
Reviewed-on: http://gerrit.openafs.org/4859
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>