runtime: fix checkmark scanning of finalizers

Currently work.finalizersDone is reset only at the beginning of
gcStart. As a result, it will be set when checkmark runs, so checkmark
will skip scanning finalizers. Hence, if there are any bugs that cause
the regular scan of finalizers to miss pointers, checkmark will also
miss them and fail to detect the missed pointer.

Fix this by resetting finalizersDone in gcResetMarkState. This way it
gets reset before any full mark, which is exactly what we want.

Change-Id: I4ddb5eba5b3b97e52aaf3e08fd9aa692bda32b20
Reviewed-on: https://go-review.googlesource.com/20332
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/mgc.go

@@ -949,8 +949,6 @@ func gcStart(mode gcMode, forceTrigger bool) {
 	// reclaimed until the next GC cycle.
 	clearpools()
 
-	work.finalizersDone = false
-
 	if mode == gcBackgroundMode { // Do as much work concurrently as possible
 		gcController.startCycle()
 		work.heapGoal = gcController.heapGoal
@@ -1738,6 +1736,7 @@ func gcResetMarkState() {
 
 	work.bytesMarked = 0
 	work.initialHeapLive = memstats.heap_live
+	work.finalizersDone = false
 }
 
 // Hooks for other packages