We have a number of netlink commands in the ethnl family that may have
multiple objects to dump even for a single net_device, including :

 - PLCA, PSE-PD, phy: one message per PHY device
 - tsinfo: one message per timestamp source (netdev + phys)
 - rss: One per RSS context

To get this behaviour, these netlink commands need to roll a custom
->dumpit().  

To prepare making per-netdev DUMP more generic in ethnl, introduce a
member in the ethnl ops to indicate if a given command may allow
pernetdev DUMPs (also referred to as filtered DUMPs).

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
---
V2: - Rebase
    - Fix kdoc
    - Fix bissectabilitu by using the right function names

 net/ethtool/netlink.c | 45 ++++++++++++++++++++++++++++---------------
 net/ethtool/netlink.h |  2 ++
 2 files changed, 31 insertions(+), 16 deletions(-)

diff --git a/net/ethtool/netlink.c b/net/ethtool/netlink.c
index 70834947f474..11e4122b7707 100644
--- a/net/ethtool/netlink.c
+++ b/net/ethtool/netlink.c

@@ -586,21 +586,34 @@ static int ethnl_default_dumpit(struct sk_buff *skb,
 	int ret = 0;
 
 	rcu_read_lock();
-	for_each_netdev_dump(net, dev, ctx->pos_ifindex) {
-		dev_hold(dev);
+	if (ctx->req_info->dev) {
+		dev = ctx->req_info->dev;
 		rcu_read_unlock();
-
-		ret = ethnl_default_dump_one(skb, dev, ctx,

genl_info_dump(cb)); -
+		/* Filtered DUMP request targeted to a single netdev. We
already
+		 * hold a ref to the netdev from ->start()
+		 */
+		ret = ethnl_default_dump_one(skb, dev, ctx,
+					     genl_info_dump(cb));
 		rcu_read_lock();
-		dev_put(dev);
-
-		if (ret < 0 && ret != -EOPNOTSUPP) {
-			if (likely(skb->len))
-				ret = skb->len;
-			break;