prevent rx peer timeout from reaching 0.0 seconds

prevent rx peer timeout from reaching 0.0 seconds

The rx peer timeout is computed from the round trip time
calculation.  It traditionally has had a lowerbound of 350ms.
The computation in rxi_ComputeRoundTripTime() was incorrect
and instead used 350ms as an upperbound.

rxi_ComputeRoundTripTime() had a second problem wherein if
the actually RTT is shorter than the resolution of the clock
then the RTT would quickly approach 0.0 seconds.  Enforce
a lowerbound of 1ms if the RTT for a given packet appears
to be 0.0 seconds.

LICENSE BSD

Reviewed-on: http://gerrit.openafs.org/696
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>