shell/commands: fix, only accept proper pong response to icmpv6_echo

[fjmolinas]

Contribution description

This PR fixes the Bug introduced in #11933.

Testing procedure

Follow the testing procedure in #11519. To test in a controlled setting follow the testing procedure sugested by @benemorius:

To test this in a controlled setting I captured and injected ping replies with a Linux node connected via a border router to a Riot node using this procedure:

1. Prepare Linux node to capture its own outgoing ping reply:
   tcpdump -i eth0 "icmp6 && ip6[40] == 129" -n -c1 -w icmp-reply.pcap
2. Use the border router to block the genuine ping reply from reaching the Riot node:
   ip6tables -I FORWARD -o lowpan0 -p icmpv6 --icmpv6-type echo-reply -j DROP
3. Ping Linux node from Riot node using 1 ping and a 60 second timeout:
   ping6 2001:470:4bb0:ffff::1 -c 1 -W 60000
4. On Linux node modify the source address of the captured packet:
   tcprewrite --infile=icmp-reply.pcap --outfile=icmp-reply2.pcap --pnat=[2001:470:4bb0:ffff::1]:[2001:470:4bb0:ffff::2]
5. On border router unblock ping replies:
   ip6tables -D FORWARD -o lowpan0 -p icmpv6 --icmpv6-type echo-reply -j DROP
6. On Linux node send modified ping reply:
   tcpreplay --intf1=eth0 icmp-reply2.pcap
7. Observe whether the Riot node prints the reply. It should not.
   12 bytes from 2001:470:4bb0:ffff::2 id:0xbe45/0xbe45 icmp_seq=0 ttl=63 rssi=-85 dBm time=13823.374 ms
8. As before, block ping replies at the border router:
   ip6tables -I FORWARD -o lowpan0 -p icmpv6 --icmpv6-type echo-reply -j DROP
9. As before, send a ping from the Riot node:
   ping6 2001:470:4bb0:ffff::1 -c 1 -W 60000
10. As before, unblock ping replies:
    ip6tables -D FORWARD -o lowpan0 -p icmpv6 --icmpv6-type echo-reply -j DROP
11. From the Linux node send the unmodified ping reply captured earlier:
    tcpreplay --intf1=eth0 icmp-reply.pcap
12. Observe whether the Riot node prints the reply. It should not.
    12 bytes from 2001:470:4bb0:ffff::1 id:0xbe45/0x2e08 icmp_seq=0 ttl=63 rssi=-86 dBm time=52156.860 ms

Issues/PRs references

Fixes #11519

[miri64]

Ooops, ACK!

[miri64]

Foiled by deMorgans law once again ;-).

[maribu]

Thanks for the fix!

[benpicco]

With this, ping6 ff02::1 will not print any response 😕