runtime: use correct state machine in addAdjustedTimers

The addAdjustedTimers function was a late addition, and it got some of
the state machine wrong, leading to failures like
https://storage.googleapis.com/go-build-log/930576b6/windows-amd64-2016_53d0319e.log

Updates #6239
Updates #27707

Change-Id: I9e94e563b4698ff3035ce609055ca292b9cab3df
Reviewed-on: https://go-review.googlesource.com/c/go/+/204280
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/time.go

@@ -989,10 +989,12 @@ func addAdjustedTimers(pp *p, moved []*timer) {
 			case timerDeleted:
 				// Timer has been deleted since we adjusted it.
 				// This timer is already out of the heap.
-				if !atomic.Cas(&t.status, s, timerRemoved) {
-					badTimer()
+				if atomic.Cas(&t.status, s, timerRemoving) {
+					if !atomic.Cas(&t.status, timerRemoving, timerRemoved) {
+						badTimer()
+					}
+					break loop
 				}
-				break loop
 			case timerModifiedEarlier, timerModifiedLater:
 				// Timer has been modified again since
 				// we adjusted it.
@@ -1007,8 +1009,8 @@ func addAdjustedTimers(pp *p, moved []*timer) {
 					if s == timerModifiedEarlier {
 						atomic.Xadd(&pp.adjustTimers, -1)
 					}
+					break loop
 				}
-				break loop
 			case timerNoStatus, timerRunning, timerRemoving, timerRemoved, timerMoving:
 				badTimer()
 			case timerModifying: