net: fix close on closed listener on Plan 9

Since close errors have been cleaned up in CL 39997,
TestCloseError is failing on Plan 9, because
TCPListener.Close didn't check that the listener
has already been closed before writing the "hangup"
string to the listener control file.

This change fixes TCPListener.Close on Plan 9,
by closing poll.FD before writing the "hangup"
string.

Fixes #20128.

Change-Id: I13862b23a9055dd1be658acef7066707d98c591f
Reviewed-on: https://go-review.googlesource.com/41850
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/tcpsock_plan9.go

@@ -48,6 +48,9 @@ func (ln *TCPListener) accept() (*TCPConn, error) {
 }
 
 func (ln *TCPListener) close() error {
+	if err := ln.fd.pfd.Close(); err != nil {
+		return err
+	}
 	if _, err := ln.fd.ctl.WriteString("hangup"); err != nil {
 		ln.fd.ctl.Close()
 		return err