packet03: comment out unused variable

Make failed due to following error:

```
xdp_prog_kern.c:60:8: error: variable 'echo_reply' set but not used [-Werror,-Wunused-but-set-variable]
        __u16 echo_reply;
              ^
1 error generated.
make[1]: *** [../common/common.mk:111: xdp_prog_kern.o] Error 1
make[1]: Leaving directory '/home/xdp-tutorial/packet03-redirecting'
make: *** [Makefile:13: packet03-redirecting] Error 2
```

Comment out relevant lines to fix.

Signed-off-by: Hengqi Chen <chenhengqi@outlook.com>

Author: chenhengqi

packet03-redirecting/xdp_prog_kern.c

@@ -57,7 +57,7 @@ int xdp_icmp_echo_func(struct xdp_md *ctx)
 	int icmp_type;
 	struct iphdr *iphdr;
 	struct ipv6hdr *ipv6hdr;
-	__u16 echo_reply;
+	// __u16 echo_reply;
 	struct icmphdr_common *icmphdr;
 	__u32 action = XDP_PASS;
 
@@ -88,12 +88,12 @@ int xdp_icmp_echo_func(struct xdp_md *ctx)
 	if (eth_type == bpf_htons(ETH_P_IP) && icmp_type == ICMP_ECHO) {
 		/* Swap IP source and destination */
 		swap_src_dst_ipv4(iphdr);
-		echo_reply = ICMP_ECHOREPLY;
+		// echo_reply = ICMP_ECHOREPLY;
 	} else if (eth_type == bpf_htons(ETH_P_IPV6)
 		   && icmp_type == ICMPV6_ECHO_REQUEST) {
 		/* Swap IPv6 source and destination */
 		swap_src_dst_ipv6(ipv6hdr);
-		echo_reply = ICMPV6_ECHO_REPLY;
+		// echo_reply = ICMPV6_ECHO_REPLY;
 	} else {
 		goto out;
 	}