[release-branch.go1.12] internal/poll: fix deadlock in Write if len(buf) > maxRW

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

Manual backport of CL 165598. It could not be cherry-picked due to conflicts.

Fixes #31211

Change-Id: Ib76e679f6a276b32fe9c1594b7e9a506017a7967
Reviewed-on: https://go-review.googlesource.com/c/go/+/170680
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/internal/poll/fd_windows.go

@@ -660,6 +660,10 @@ func (fd *FD) Write(buf []byte) (int, error) {
 		return 0, err
 	}
 	defer fd.writeUnlock()
+	if fd.isFile || fd.isDir || fd.isConsole {
+		fd.l.Lock()
+		defer fd.l.Unlock()
+	}
 
 	ntotal := 0
 	for len(buf) > 0 {
@@ -670,8 +674,6 @@ func (fd *FD) Write(buf []byte) (int, error) {
 		var n int
 		var err error
 		if fd.isFile || fd.isDir || fd.isConsole {
-			fd.l.Lock()
-			defer fd.l.Unlock()
 			if fd.isConsole {
 				n, err = fd.writeConsole(b)
 			} else {