os: reject OpenDir of a non-directory file in Plan 9

Check that the path argument to OpenDir in Plan 9 is a directory,
and return error syscall.ENOTDIR if it is not.

Fixes #75196

Change-Id: I3bec6b6b40a38c21264b5d22ff3e7dfbf8c1c6d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/700855
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: David du Colombier <0intro@gmail.com>

src/os/file_plan9.go

@@ -135,7 +135,20 @@ func openFileNolog(name string, flag int, perm FileMode) (*File, error) {
 }
 
 func openDirNolog(name string) (*File, error) {
-	return openFileNolog(name, O_RDONLY, 0)
+	f, e := openFileNolog(name, O_RDONLY, 0)
+	if e != nil {
+		return nil, e
+	}
+	d, e := f.Stat()
+	if e != nil {
+		f.Close()
+		return nil, e
+	}
+	if !d.IsDir() {
+		f.Close()
+		return nil, &PathError{Op: "open", Path: name, Err: syscall.ENOTDIR}
+	}
+	return f, nil
 }
 
 // Close closes the File, rendering it unusable for I/O.