netfilter: nft_ct: fix random validation errors for zone set support

Dan reports:
 net/netfilter/nft_ct.c:549 nft_ct_set_init()
 error: uninitialized symbol 'len'.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: edee4f1e ("netfilter: nft_ct: add zone id set support")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_ct.c

@@ -528,6 +528,7 @@ static int nft_ct_set_init(const struct nft_ctx *ctx,
 		if (!nft_ct_tmpl_alloc_pcpu())
 			return -ENOMEM;
 		nft_ct_pcpu_template_refcnt++;
+		len = sizeof(u16);
 		break;
 #endif
 	default: