tun/netstack: remove usage of pkt.IsNil()

Since 3c75945fd ("netstack: remove PacketBuffer.IsNil()") this has been
invalid. Follow the replacement pattern of that commit.

The old definition inlined to the same code anyway:

 func (pk *PacketBuffer) IsNil() bool {
 	return pk == nil
 }

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

tun/netstack/tun.go

@@ -155,7 +155,7 @@ func (tun *netTun) Write(buf [][]byte, offset int) (int, error) {
 
 func (tun *netTun) WriteNotify() {
 	pkt := tun.ep.Read()
-	if pkt.IsNil() {
+	if pkt == nil {
 		return
 	}