donot fallback if already in fallback

Author: menghanl

balancer/grpclb/grpclb.go

@@ -364,7 +364,7 @@ func (lb *lbBalancer) fallbackToBackendsAfter(fallbackTimeout time.Duration) {
 		return
 	}
 	lb.mu.Lock()
-	if lb.serverListReceived {
+	if lb.inFallback || lb.serverListReceived {
 		lb.mu.Unlock()
 		return
 	}