Fix "irrefutable while let pattern" warning

```
warning: irrefutable `while let` pattern
  --> examples/arp_packet.rs:61:11
   |
61 |     while let buf = receiver.next().unwrap() {
   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: this pattern will always match, so the loop will never exit
   = help: consider instead using a `loop { ... }` with a `let` inside it
   = note: `#[warn(irrefutable_let_patterns)]` on by default
```

examples/arp_packet.rs

@@ -58,7 +58,8 @@ fn get_mac_through_arp(interface: NetworkInterface, target_ip: Ipv4Addr) -> MacA
 
     println!("Sent ARP request");
 
-    while let buf = receiver.next().unwrap() {
+    loop {
+        let buf = receiver.next().unwrap();
         let arp = ArpPacket::new(&buf[MutableEthernetPacket::minimum_packet_size()..]).unwrap();
         if arp.get_sender_proto_addr() == target_ip
             && arp.get_target_hw_addr() == interface.mac.unwrap()
@@ -67,7 +68,7 @@ fn get_mac_through_arp(interface: NetworkInterface, target_ip: Ipv4Addr) -> MacA
             return arp.get_sender_hw_addr();
         }
     }
-    panic!("Never reach here")
+    // unreachable
 }
 
 fn main() {