ktls: fix a panic with INVARIANTS

98215005b747fef67f44794ca64abd473b98bade introduced a new
thread that uses tsleep(..0) to sleep forever. This hit
an assert due to sleeping with a 0 timeout.

So spell "forever" using SBT_MAX instead, which does not
trigger the assert.

Pointy hat to: gallatin
Pointed out by: emaste
Sponsored by: Netflix