sched: Inline the need_resched test into the caller for _cond_resched

_cond_resched does at least two explicit calls just to decide to do
nothing: _cond_resched and should_resched(). Inline a need_resched()
into the caller to avoid these calls in the common case of no reschedule
being needed.

Signed-off-by: Andi Kleen <ak@linux.intel.com>