sched: add do_sched_yield() helper; remove in-kernel call to sched_yield()

Using the sched-internal do_sched_yield() helper allows us to get rid of
the sched-internal call to the sys_sched_yield() syscall.

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net

Cc: Ingo Molnar <mingo@redhat.com>
Acked-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: NDominik Brodowski <linux@dominikbrodowski.net>

kernel/sched/core.c

@@ -4892,7 +4892,7 @@ SYSCALL_DEFINE3(sched_getaffinity, pid_t, pid, unsigned int, len,
  *
  * Return: 0.
  */
-SYSCALL_DEFINE0(sched_yield)
+static void do_sched_yield(void)
 {
 	struct rq_flags rf;
 	struct rq *rq;
@@ -4913,7 +4913,11 @@ SYSCALL_DEFINE0(sched_yield)
 	sched_preempt_enable_no_resched();
 
 	schedule();
+}
 
+SYSCALL_DEFINE0(sched_yield)
+{
+	do_sched_yield();
 	return 0;
 }
 
@@ -4997,7 +5001,7 @@ EXPORT_SYMBOL(__cond_resched_softirq);
 void __sched yield(void)
 {
 	set_current_state(TASK_RUNNING);
-	sys_sched_yield();
+	do_sched_yield();
 }
 EXPORT_SYMBOL(yield);