internal/poll: only call SetFileCompletionNotificationModes for sockets

CL 36799 made SetFileCompletionNotificationModes to be called for
file handles. I don't think it is correct. Revert that change.

Fixes #22024
Fixes #22207

Change-Id: I26260e8a727131cffbf60958d79eca2457495554
Reviewed-on: https://go-review.googlesource.com/69871Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/internal/poll/fd_windows.go

@@ -357,12 +357,12 @@ func (fd *FD) Init(net string, pollable bool) (string, error) {
 	if err != nil {
 		return "", err
 	}
-	if useSetFileCompletionNotificationModes {
+	if pollable && useSetFileCompletionNotificationModes {
 		// We do not use events, so we can skip them always.
 		flags := uint8(syscall.FILE_SKIP_SET_EVENT_ON_HANDLE)
 		// It's not safe to skip completion notifications for UDP:
 		// http://blogs.technet.com/b/winserverperformance/archive/2008/06/26/designing-applications-for-high-performance-part-iii.aspx
-		if net == "tcp" || net == "file" {
+		if net == "tcp" {
 			flags |= syscall.FILE_SKIP_COMPLETION_PORT_ON_SUCCESS
 		}
 		err := syscall.SetFileCompletionNotificationModes(fd.Sysfd, flags)