unix: add IoctlGetHwTstamp/IoctlGetHwTstamp on Linux

Add the ioctls to get/set the hardware timestamping configuration
(SIOCSHWTSTAMP and SIOCGHWTSTAMP) along with relevant symbols.

The usage is described in
https://www.kernel.org/doc/Documentation/networking/timestamping.txt

Change-Id: Ib7509feaf28218aeae497eff9ca6c0a532aa73c0
GitHub-Last-Rev: 47dc9dfec291d2b8f0c3df3aca1b3d3d510baecb
GitHub-Pull-Request: golang/sys#224
Reviewed-on: https://go-review.googlesource.com/c/sys/+/620376
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

unix/ioctl_linux.go

@@ -73,6 +73,32 @@ func IoctlGetEthtoolTsInfo(fd int, ifname string) (*EthtoolTsInfo, error) {
 	return &value, err
 }
 
+// IoctlGetHwTstamp retrieves the hardware timestamping configuration
+// for the network device specified by ifname.
+func IoctlGetHwTstamp(fd int, ifname string) (*HwTstampConfig, error) {
+	ifr, err := NewIfreq(ifname)
+	if err != nil {
+		return nil, err
+	}
+
+	value := HwTstampConfig{}
+	ifrd := ifr.withData(unsafe.Pointer(&value))
+
+	err = ioctlIfreqData(fd, SIOCGHWTSTAMP, &ifrd)
+	return &value, err
+}
+
+// IoctlSetHwTstamp updates the hardware timestamping configuration for
+// the network device specified by ifname.
+func IoctlSetHwTstamp(fd int, ifname string, cfg *HwTstampConfig) error {
+	ifr, err := NewIfreq(ifname)
+	if err != nil {
+		return err
+	}
+	ifrd := ifr.withData(unsafe.Pointer(cfg))
+	return ioctlIfreqData(fd, SIOCSHWTSTAMP, &ifrd)
+}
+
 // IoctlGetWatchdogInfo fetches information about a watchdog device from the
 // Linux watchdog API. For more information, see:
 // https://www.kernel.org/doc/html/latest/watchdog/watchdog-api.html.