netfilter: core: return EBUSY in case NAT hook is already in use

EEXIST is used for an object that already exists, with the same
name/handle. However, there no same object there, instead there is a
object that is using the single slot that is available for NAT hooks
since patch f92b40a8b2 ("netfilter: core: only allow one nat hook per
hook point"). Let's change this return value before this behaviour gets
exposed in the first -rc.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/core.c

@@ -140,7 +140,7 @@ nf_hook_entries_grow(const struct nf_hook_entries *old,
 
 		if (reg->nat_hook && orig_ops[i]->nat_hook) {
 			kvfree(new);
-			return ERR_PTR(-EEXIST);
+			return ERR_PTR(-EBUSY);
 		}
 
 		if (inserted || reg->priority > orig_ops[i]->priority) {