netfilter: nf_nat: fix inverted logic for persistent NAT mappings

Kernel 2.6.30 introduced a patch [1] for the persistent option in the
netfilter SNAT target. This is exactly what we need here so I had a quick look
at the code and noticed that the patch is wrong. The logic is simply inverted.
The patch below fixes this.

Also note that because of this the default behavior of the SNAT target has
changed since kernel 2.6.30 as it now ignores the destination IP in choosing
the source IP for nating (which should only be the case if the persistent
option is set).

[1] http://git.eu.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=98d500d66cb7940747b424b245fc6a51ecfbf005

Signed-off-by: Maximilian Engelhardt <maxi@daemonizer.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>

net/ipv4/netfilter/nf_nat_core.c

@@ -212,7 +212,7 @@ find_best_ips_proto(struct nf_conntrack_tuple *tuple,
 	maxip = ntohl(range->max_ip);
 	j = jhash_2words((__force u32)tuple->src.u3.ip,
 			 range->flags & IP_NAT_RANGE_PERSISTENT ?
-				(__force u32)tuple->dst.u3.ip : 0, 0);
+				0 : (__force u32)tuple->dst.u3.ip, 0);
 	j = ((u64)j * (maxip - minip + 1)) >> 32;
 	*var_ipp = htonl(minip + j);
 }