[$] Migration disable for the mainline

The realtime developers have been working for many years tocreate a kernel where the highest-priority task is always able to runwithout delay. That has meant a long process of finding and fixingsituations where high-priority tasks might be blocked from running; one ofthe persistent problems in this regard has been kernel code that disablespreemption. One tool that the realtime developers have reached for isdisabling migration (moving a process from one CPU to another) rather thanpreemption; this approach has not been entirely popular among schedulerdevelopers, though. Even so, the solution would appear to be thismigration-disable patch set from scheduler developer Peter Zijlstra.