sched: Replace rq::wake_list

The recent commit: 90b5363acd ("sched: Clean up scheduler_ipi()")
got smp_call_function_single_async() subtly wrong. Even though it will
return -EBUSY when trying to re-use a csd, that condition is not
atomic and still requires external serialization.

The change in ttwu_queue_remote() got this wrong.

While on first reading ttwu_queue_remote() has an atomic test-and-set
that appears to serialize the use, the matching 'release' is not in
the right place to actually guarantee this serialization.

The actual race is vs the sched_ttwu_pending() call in the idle loop;
that can run the wakeup-list without consuming the CSD.

Instead of trying to chain the lists, merge them.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200526161908.129371594@infradead.org

include/linux/smp.h

@@ -25,6 +25,7 @@ enum {
 	CSD_TYPE_ASYNC		= 0x00,
 	CSD_TYPE_SYNC		= 0x10,
 	CSD_TYPE_IRQ_WORK	= 0x20,
+	CSD_TYPE_TTWU		= 0x30,
 	CSD_FLAG_TYPE_MASK	= 0xF0,
 };