runtime: scanvalid race Fixes #9727

Set gcscanvalid=false after you have cased to _Grunning.
If you do it before the cas and the atomicstatus races to a scan state,
the scan will set gcscanvalid=true and we will be _Grunning
with gcscanvalid==true which is not a good thing.

Change-Id: Ie53ea744a5600392b47da91159d985fe6fe75961
Reviewed-on: https://go-review.googlesource.com/3510Reviewed-by: Austin Clements <austin@google.com>

src/runtime/proc1.go

@@ -387,10 +387,6 @@ func casgstatus(gp *g, oldval, newval uint32) {
 		})
 	}
 
-	if newval == _Grunning {
-		gp.gcscanvalid = false
-	}
-
 	// loop if gp->atomicstatus is in a scan state giving
 	// GC time to finish and change the state to oldval.
 	for !cas(&gp.atomicstatus, oldval, newval) {
@@ -407,6 +403,9 @@ func casgstatus(gp *g, oldval, newval uint32) {
 		// 	})
 		// }
 	}
+	if newval == _Grunning {
+		gp.gcscanvalid = false
+	}
 }
 
 // casgstatus(gp, oldstatus, Gcopystack), assuming oldstatus is Gwaiting or Grunnable.