netfilter: nat: fix ICMP header corruption on ICMP errors

netfilter: nat: fix ICMP header corruption on ICMP errors

Commit 8303b7e8f018 ("netfilter: nat: fix spurious connection timeouts")
made nf_nat_icmp_reply_translation() use icmp_manip_pkt() as the l4
manipulation function for the outer packet on ICMP errors.

However, icmp_manip_pkt() assumes the packet has an 'id' field which
is not correct for all types of ICMP messages.

This is not correct for ICMP error packets, and leads to bogus bytes
being written the ICMP header, which can be wrongfully regarded as
'length' bytes by RFC 4884 compliant receivers.

Fix by assigning the 'id' field only for ICMP messages that have this
semantic.

Reported-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Fixes: 8303b7e8f018 ("netfilter: nat: fix spurious connection timeouts")
Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>