netfilter: restore POST_ROUTING hook in NF_HOOK_COND

Commit 2249065f ("netfilter: get rid of the grossness in netfilter.h")
inverted the logic for conditional hook invocation, breaking the
POST_ROUTING hook invoked by ip_output().

Correct the logic and remove an unnecessary initialization.
Reported-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>

include/linux/netfilter.h

@@ -212,8 +212,9 @@ NF_HOOK_COND(uint8_t pf, unsigned int hook, struct sk_buff *skb,
 	     struct net_device *in, struct net_device *out,
 	     int (*okfn)(struct sk_buff *), bool cond)
 {
-	int ret = 1;
-	if (cond ||
+	int ret;
+
+	if (!cond ||
 	    (ret = nf_hook_thresh(pf, hook, skb, in, out, okfn, INT_MIN) == 1))
 		ret = okfn(skb);
 	return ret;