runtime: prevent sysmon from polling network excessivly

If the network is not polled for 10ms, sysmon starts polling network
on every iteration (every 20us) until another thread blocks in netpoll.
Fixes #5922.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/11569043

src/pkg/runtime/proc.c

@@ -2098,6 +2098,7 @@ sysmon(void)
 		lastpoll = runtime·atomicload64(&runtime·sched.lastpoll);
 		now = runtime·nanotime();
 		if(lastpoll != 0 && lastpoll + 10*1000*1000 > now) {
+			runtime·cas64(&runtime·sched.lastpoll, lastpoll, now);
 			gp = runtime·netpoll(false);  // non-blocking
 			injectglist(gp);
 		}