runtime: don't return from netpollGenericInit until init is complete

As a side-effect ensure that netpollinited only reports true when
netpoll initialization is complete.

Fixes #35282
Updates #35353

Change-Id: I21f08a04fcf229e0de5e6b5ad89c990426ae9b89
Reviewed-on: https://go-review.googlesource.com/c/go/+/204937
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/netpoll.go

@@ -93,7 +93,9 @@ type pollCache struct {
 }
 
 var (
-	netpollInited  uint32
+	netpollInitLock mutex
+	netpollInited   uint32
+
 	pollcache      pollCache
 	netpollWaiters uint32
 )
@@ -104,8 +106,13 @@ func poll_runtime_pollServerInit() {
 }
 
 func netpollGenericInit() {
-	if atomic.Cas(&netpollInited, 0, 1) {
-		netpollinit()
+	if atomic.Load(&netpollInited) == 0 {
+		lock(&netpollInitLock)
+		if netpollInited == 0 {
+			netpollinit()
+			atomic.Store(&netpollInited, 1)
+		}
+		unlock(&netpollInitLock)
 	}
 }