sched_ule(4): Fix hang with steal_thresh < 2.
e745d729 caused infinite loop with interrupts disabled in load stealing code if steal_thresh set below 2. Such configuration should not generally be used, but appeared some people are using it to workaround some problems. To fix the problem explicitly pass to sched_highest() minimum number of transferrable threads, supported by the caller, instead of guessing. MFC after: 25 days