syscall: dup the argument to fdopendir

fdopendir takes ownership of its file descriptor argument.
Getdirentries shouldn't do that, so dup the file descriptor
before passing to fdopendir.

Fixes #31269

Change-Id: Ie36be8fd6c59eb339dcc9f40228d4191fc1e5850
Reviewed-on: https://go-review.googlesource.com/c/go/+/170698
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/syscall/syscall_darwin.go

@@ -368,10 +368,15 @@ func writelen(fd int, buf *byte, nbuf int) (n int, err error) {
 func Getdirentries(fd int, buf []byte, basep *uintptr) (n int, err error) {
 	// Simulate Getdirentries using fdopendir/readdir_r/closedir.
 	const ptrSize = unsafe.Sizeof(uintptr(0))
-	d, err := fdopendir(fd)
+	fd2, err := Dup(fd)
 	if err != nil {
 		return 0, err
 	}
+	d, err := fdopendir(fd2)
+	if err != nil {
+		Close(fd2)
+		return 0, err
+	}
 	defer closedir(d)
 	// We keep the number of records already returned in *basep.
 	// It's not the full required semantics, but should handle the case