netfilter: nf_conntrack_irc: Fix forged IP logic

netfilter: nf_conntrack_irc: Fix forged IP logic

[ Upstream commit 3ab5885c831c717729e0799aa292f2ecc0484ce4 ]

Ensure the match happens in the right direction, previously the
destination used was the server, not the NAT host, as the comment
shows the code intended.

Additionally nf_nat_irc uses port 0 as a signal and there's no valid way
it can appear in a DCC message, so consider port 0 also forged.

Fixes: f8a64fc724f9 ("[NETFILTER]: nf_conntrack/nf_nat: add IRC helper port")
Signed-off-by: David Leadbeater <dgl@dgl.cx>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>