netfilter: ebtables: compat: un-break 32bit setsockopt when no rules are present

Unlike ip(6)tables ebtables only counts user-defined chains.

The effect is that a 32bit ebtables binary on a 64bit kernel can do
'ebtables -N FOO' only after adding at least one rule, else the request
fails with -EINVAL.

This is a similar fix as done in
3f1e53ab ("netfilter: ebtables: don't attempt to allocate 0-sized compat array").

Fixes: 7d7d7e02 ("netfilter: compat: reject huge allocation requests")
Reported-by: Francesco Ruggeri <fruggeri@arista.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/bridge/netfilter/ebtables.c

@@ -2293,9 +2293,12 @@ static int compat_do_replace(struct net *net, void __user *user,
 
 	xt_compat_lock(NFPROTO_BRIDGE);
 
-	ret = xt_compat_init_offsets(NFPROTO_BRIDGE, tmp.nentries);
-	if (ret < 0)
-		goto out_unlock;
+	if (tmp.nentries) {
+		ret = xt_compat_init_offsets(NFPROTO_BRIDGE, tmp.nentries);
+		if (ret < 0)
+			goto out_unlock;
+	}
+
 	ret = compat_copy_entries(entries_tmp, tmp.entries_size, &state);
 	if (ret < 0)
 		goto out_unlock;