internal/poll: fix deadlock in Write if len(buf) > maxRW

fd.l.Lock shouldn't be called in a loop.

Change-Id: I3afbc184aa06a60175c9a39319985b5810ecb144
Reviewed-on: https://go-review.googlesource.com/c/go/+/165598
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/internal/poll/fd_windows.go

@@ -673,6 +673,10 @@ func (fd *FD) Write(buf []byte) (int, error) {
 		return 0, err
 	}
 	defer fd.writeUnlock()
+	if fd.isFile {
+		fd.l.Lock()
+		defer fd.l.Unlock()
+	}
 
 	ntotal := 0
 	for len(buf) > 0 {
@@ -683,8 +687,6 @@ func (fd *FD) Write(buf []byte) (int, error) {
 		var n int
 		var err error
 		if fd.isFile {
-			fd.l.Lock()
-			defer fd.l.Unlock()
 			switch fd.kind {
 			case kindConsole:
 				n, err = fd.writeConsole(b)