sched/core: Use put_prev_task() accessor where possible

Signed-off-by: Kirill Tkhai <ktkhai@parallels.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1410529300.3569.25.camel@tkhai


Signed-off-by: Ingo Molnar <mingo@kernel.org>