runtime: reset sweep stats before starting the world

Currently we reset the sweep stats just after gcMarkTermination starts
the world and releases worldsema. However, background sweeping can
start the moment we start the world and, in fact, pause sweeping can
start the moment we release worldsema (because another GC cycle can
start up), so these need to be cleared before starting the world.

Change-Id: I95701e3de6af76bb3fbf2ee65719985bf57d20b2
Reviewed-on: https://go-review.googlesource.com/17811
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/mgc.go

@@ -1245,6 +1245,10 @@ func gcMarkTermination() {
 
 	memstats.numgc++
 
+	// Reset sweep state.
+	sweep.nbgsweep = 0
+	sweep.npausesweep = 0
+
 	systemstack(startTheWorldWithSema)
 
 	// Free stack spans. This must be done between GC cycles.
@@ -1301,8 +1305,6 @@ func gcMarkTermination() {
 		print("\n")
 		printunlock()
 	}
-	sweep.nbgsweep = 0
-	sweep.npausesweep = 0
 
 	// now that gc is done, kick off finalizer thread if needed
 	if !concurrentSweep {