Instead of dealing with sched classes inside each check_preempt_curr()
implementation, pull out this logic into the generic wakeup preemption
path.

This fixes a hang in KVM (and others) where we are waiting for the
stop machine thread to run ...
Reported-by: NMarkus Trippelsdorf <markus@trippelsdorf.de>
Tested-by: NMarcelo Tosatti <mtosatti@redhat.com>
Tested-by: NSergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: NPeter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1288891946.2039.31.camel@laptop>
Signed-off-by: NIngo Molnar <mingo@elte.hu>