netfilter: core: remove erroneous warn_on

kernel test robot reported:

WARNING: CPU: 0 PID: 1244 at net/netfilter/core.c:218 __nf_hook_entries_try_shrink+0x49/0xcd
[..]

After allowing batching in nf_unregister_net_hooks its possible that an earlier
call to __nf_hook_entries_try_shrink already compacted the list.
If this happens we don't need to do anything.

Fixes: d3ad2c17 ("netfilter: core: batch nf_unregister_net_hooks synchronize_net calls")
Reported-by: kernel test robot <xiaolong.ye@intel.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Aaron Conole <aconole@bytheb.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/core.c

@@ -215,7 +215,7 @@ static void *__nf_hook_entries_try_shrink(struct nf_hook_entries __rcu **pp)
 	if (skip == hook_entries)
 		goto out_assign;
 
-	if (WARN_ON(skip == 0))
+	if (skip == 0)
 		return NULL;
 
 	hook_entries -= skip;