Fix handling of verdicts after NF_QUEUE

[This fix is only needed for v4.9 stable since v4.10+ does not have the issue]

A verdict of NF_STOLEN after NF_QUEUE will cause an incorrect return value
and a potential kernel panic via double free of skb's

This was broken by commit 7034b566a4 ("netfilter: fix nf_queue handling")
and subsequently fixed in v4.10 by commit c63cbc4604 ("netfilter:
use switch() to handle verdict cases from nf_hook_slow()"). However that
commit cannot be cleanly cherry-picked to v4.9

Signed-off-by: Debabrata Banerjee <dbanerje@akamai.com>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/core.c

@@ -364,6 +364,11 @@ next_hook:
 		ret = nf_queue(skb, state, &entry, verdict);
 		if (ret == 1 && entry)
 			goto next_hook;
+	} else {
+		/* Implicit handling for NF_STOLEN, as well as any other
+		 * non conventional verdicts.
+		 */
+		ret = 0;
 	}
 	return ret;
 }