netfilter: NF_HOOK_COND has wrong conditional

The NF_HOOK_COND returns 0 when it shouldn't due to what I believe to be an
error in the code as the order of operations is not what was intended.  C will
evalutate == before =.  Which means ret is getting set to the bool result,
rather than the return value of the function call.  The code says

if (ret = function() == 1)
when it meant to say:
if ((ret = function()) == 1)

Normally the compiler would warn, but it doesn't notice it because its
a actually complex conditional and so the wrong code is wrapped in an explict
set of () [exactly what the compiler wants you to do if this was intentional].
Fixing this means that errors when netfilter denies a packet get propagated
back up the stack rather than lost.

Problem introduced by commit 2249065f (netfilter: get rid of the grossness
in netfilter.h).
Signed-off-by: Eric Paris <eparis@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Patrick McHardy <kaber@trash.net>

include/linux/netfilter.h

@@ -215,7 +215,7 @@ NF_HOOK_COND(uint8_t pf, unsigned int hook, struct sk_buff *skb,
 	int ret;
 
 	if (!cond ||
-	    (ret = nf_hook_thresh(pf, hook, skb, in, out, okfn, INT_MIN) == 1))
+	    ((ret = nf_hook_thresh(pf, hook, skb, in, out, okfn, INT_MIN)) == 1))
 		ret = okfn(skb);
 	return ret;
 }