sched: do not keep current in the tree and get rid of sched_entity::fair_key

Get rid of 'sched_entity::fair_key'.

As a side effect, 'current' is not kept withing the tree for 
SCHED_NORMAL/BATCH tasks anymore. This simplifies some parts of code 
(e.g. entity_tick() and yield_task_fair()) and also somewhat optimizes 
them (e.g. a single update_curr() now vs. dequeue/enqueue() before in 
entity_tick()).

Signed-off-by: Dmitry Adamushko <dmitry.adamushko@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
diff --git a/kernel/sched_debug.c b/kernel/sched_debug.c
index e3b6232..bb34b81 100644
--- a/kernel/sched_debug.c
+++ b/kernel/sched_debug.c
@@ -38,7 +38,7 @@
 
 	SEQ_printf(m, "%15s %5d %15Ld %13Ld %5d ",
 		p->comm, p->pid,
-		(long long)p->se.fair_key,
+		(long long)p->se.vruntime,
 		(long long)(p->nvcsw + p->nivcsw),
 		p->prio);
 #ifdef CONFIG_SCHEDSTATS