Rack conversion in host cache needs to be conditional on the host cache being used.

[rrs]

Summary

Looking at the hostcache conversion it is subject to when we do *not* have an RTT. So
in such a case rack could in theory incorrectly convert its already converted rtt/rrtvar and
mess up the initial RTT. We instead need to grab the srtt before we grab entries
from the host cache, and only if we got something from the hc do the update.

Test Plan

Test with pkt-drill to make sure when the srtt is non-zero we stick with the old value.s