net: fix setKeepAlivePeriod on Plan 9

The interface to set TCP keepalive on Plan 9 is
writing the "keepalive n" string to the TCP ctl file,
where n is the milliseconds between keepalives.

Fixes #11266.

Change-Id: Ic96f6c584063665a1ddf921a9a4ddfa13cc7501b
Reviewed-on: https://go-review.googlesource.com/11860
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/net/tcpsockopt_plan9.go

@@ -7,12 +7,13 @@
 package net
 
 import (
+	"strconv"
 	"time"
 )
 
 // Set keep alive period.
 func setKeepAlivePeriod(fd *netFD, d time.Duration) error {
-	cmd := "keepalive " + string(int64(d/time.Millisecond))
+	cmd := "keepalive " + strconv.Itoa(int(d/time.Millisecond))
 	_, e := fd.ctl.WriteAt([]byte(cmd), 0)
 	return e
 }