unix: use Go 1.21+ clear built-in

Inspired by CL 698495.

Change-Id: Ic6ff68a60d41b6ffb6a28f523597c5d36c1e4d2f
Reviewed-on: https://go-review.googlesource.com/c/sys/+/704915
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Mark Freeman <markfreeman@google.com>

unix/ifreq_linux.go

@@ -111,9 +111,7 @@ func (ifr *Ifreq) SetUint32(v uint32) {
 // clear zeroes the ifreq's union field to prevent trailing garbage data from
 // being sent to the kernel if an ifreq is reused.
 func (ifr *Ifreq) clear() {
-	for i := range ifr.raw.Ifru {
-		ifr.raw.Ifru[i] = 0
-	}
+	clear(ifr.raw.Ifru[:])
 }
 
 // TODO(mdlayher): export as IfreqData? For now we can provide helpers such as