unix: add Ioctl{Get,Set}IfreqMTU on darwin

For golang/go#41868

Change-Id: I9fb8f5eee933488e4c63aaa757059b8cea4d9629
Reviewed-on: https://go-review.googlesource.com/c/sys/+/264637
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>

unix/syscall_darwin.go

@@ -264,6 +264,29 @@ func IoctlCtlInfo(fd int, ctlInfo *CtlInfo) error {
 	return err
 }
 
+// IfreqMTU is struct ifreq used to get or set a network device's MTU.
+type IfreqMTU struct {
+	Name [IFNAMSIZ]byte
+	MTU  int32
+}
+
+// IoctlGetIfreqMTU performs the SIOCGIFMTU ioctl operation on fd to get the MTU
+// of the network device specified by ifname.
+func IoctlGetIfreqMTU(fd int, ifname string) (*IfreqMTU, error) {
+	var ifreq IfreqMTU
+	copy(ifreq.Name[:], ifname)
+	err := ioctl(fd, SIOCGIFMTU, uintptr(unsafe.Pointer(&ifreq)))
+	return &ifreq, err
+}
+
+// IoctlSetIfreqMTU performs the SIOCSIFMTU ioctl operation on fd to set the MTU
+// of the network device specified by ifreq.Name.
+func IoctlSetIfreqMTU(fd int, ifreq *IfreqMTU) error {
+	err := ioctl(fd, SIOCSIFMTU, uintptr(unsafe.Pointer(ifreq)))
+	runtime.KeepAlive(ifreq)
+	return err
+}
+
 //sys   sysctl(mib []_C_int, old *byte, oldlen *uintptr, new *byte, newlen uintptr) (err error) = SYS_SYSCTL
 
 func Uname(uname *Utsname) error {