sched_ule(4): Fix hang with steal_thresh < 2.

e745d729be60 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