sched: improve prev_sum_exec_runtime setting

Second preparatory patch for fix-ideal runtime:

Mark prev_sum_exec_runtime at the beginning of our run, the same spot
that adds our wait period to wait_runtime. This seems a more natural
location to do this, and it also reduces the code a bit:

   text    data     bss     dec     hex filename
  13397     228    1204   14829    39ed sched.o.before
  13391     228    1204   14823    39e7 sched.o.after

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