tcp: Rack idle reduce not working.

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.

Reviewed by: tuexen, thj
Sponsored by: Netflix Inc
Differential Revision: https://reviews.freebsd.org/D35066