genetlink: factor skb preparation out of ctrl_dumppolicy()

We'll need this later for the per-op policy index dump.
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netlink/genetlink.c

@@ -1154,21 +1154,34 @@ static int ctrl_dumppolicy_start(struct netlink_callback *cb)
 					      rt->maxattr);
 }
 
-static int ctrl_dumppolicy(struct sk_buff *skb, struct netlink_callback *cb)
+static void *ctrl_dumppolicy_prep(struct sk_buff *skb,
+				  struct netlink_callback *cb)
 {
 	struct ctrl_dump_policy_ctx *ctx = (void *)cb->ctx;
-
-	while (netlink_policy_dump_loop(ctx->state)) {
 	void *hdr;
-		struct nlattr *nest;
 
 	hdr = genlmsg_put(skb, NETLINK_CB(cb->skb).portid,
 			  cb->nlh->nlmsg_seq, &genl_ctrl,
 			  NLM_F_MULTI, CTRL_CMD_GETPOLICY);
 	if (!hdr)
-			goto nla_put_failure;
+		return NULL;
 
 	if (nla_put_u16(skb, CTRL_ATTR_FAMILY_ID, ctx->fam_id))
+		return NULL;
+
+	return hdr;
+}
+
+static int ctrl_dumppolicy(struct sk_buff *skb, struct netlink_callback *cb)
+{
+	struct ctrl_dump_policy_ctx *ctx = (void *)cb->ctx;
+
+	while (netlink_policy_dump_loop(ctx->state)) {
+		void *hdr;
+		struct nlattr *nest;
+
+		hdr = ctrl_dumppolicy_prep(skb, cb);
+		if (!hdr)
 			goto nla_put_failure;
 
 		nest = nla_nest_start(skb, CTRL_ATTR_POLICY);