tcp: Rack idle reduce not working.

[rrs]

Summary

Rack converted to micro-seconds quite some time ago, but in testing
we have found a miss in that work. The idle reduce time is still based
in ticks, so it must be converted to microseconds before any comparisons
else you will likely not do idle reduce.

Test Plan

Validate that the idle reduce does work in our test lab... which it does with
this patch ;)

[thj]

Looks good to me