rx: Make rx_softAckDelay & rx_lastAckDelay private

rx: Make rx_softAckDelay & rx_lastAckDelay private

The values of these two parameters directly affect the modifiers
that are needed in the peer's RTT calculations, and so can not
arbitrarily be changed by applications.

lastAckDelay has been 400ms since the first OpenAFS release, and
that value is used as a modifier when computing the timeout of the
last packet. It is likely that any change which made this value
longer than 400ms would have detrimental effects on deployed clients

softAckDelay has been 100ms for a similar time period. We have
chopped and changed the value of minPeerTimeout, so it is unclear
what the maximal value for this parameter is. For much of OpenAFS's
life, minPeerTimeout was a 350ms padding value, which suggests that
copying TCP, and setting the maximal value at 200ms would be a safe
option. For now, however, leave it at 100ms to avoid unexpected
side effects.

hardAckDelay is not addressed by this patch set, as all ACK packets
sent from the application thread are marked as delayed, and so
currently have no part in computing RTT times. It is likely, however,
that any changes to the hard ACK timeout should be very carefully
considered.

Change-Id: Ibdeae4577b09d120f22bb922697f92d63aaf0dff
Reviewed-on: http://gerrit.openafs.org/4858
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>