sched: simplify __check_preempt_curr_fair()

Preparatory patch for fix-ideal-runtime:

simplify __check_preempt_curr_fair(): get rid of the integer return.

   text    data     bss     dec     hex filename
  13404     228    1204   14836    39f4 sched.o.before
  13393     228    1204   14825    39e9 sched.o.after

functionality is unchanged.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>