netlink: support extack in dump ->start()

Commit 4a19edb6 ("netlink: Pass extack to dump handlers")
added extack support to netlink dumps. It was focused on rtnl
and since rtnl does not use ->start(), ->done() callbacks
it ignored those. Genetlink on the other hand uses ->start()
extensively, for parsing and input validation.

Pass the extact in via struct netlink_dump_control and link
it to cb for the time of ->start(). Both struct netlink_dump_control
and extack itself live on the stack so we can't keep the same
extack for the duration of the dump. This means that the extack
visible in ->start() and each ->dump() callbacks will be different.
Corner cases like reporting a warning message in DONE across dump
calls are still not supported.

We could put the extack (for dumps) in the socket struct,
but layering makes it slightly awkward (extack pointer is decided
before the DO / DUMP split).

The genetlink dump error extacks are now surfaced:

  $ cli.py --spec netlink/specs/ethtool.yaml --dump channels-get
  lib.ynl.NlError: Netlink error: Invalid argument
  nl_len = 64 (48) nl_flags = 0x300 nl_type = 2
	error: -22	extack: {'msg': 'request header missing'}

Previously extack was missing:

  $ cli.py --spec netlink/specs/ethtool.yaml --dump channels-get
  lib.ynl.NlError: Netlink error: Invalid argument
  nl_len = 36 (20) nl_flags = 0x100 nl_type = 2
	error: -22
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netlink.h

@@ -311,6 +311,7 @@ struct netlink_dump_control {
 	int (*start)(struct netlink_callback *);
 	int (*dump)(struct sk_buff *skb, struct netlink_callback *);
 	int (*done)(struct netlink_callback *);
+	struct netlink_ext_ack *extack;
 	void *data;
 	struct module *module;
 	u32 min_dump_alloc;

net/netlink/af_netlink.c

@@ -2360,7 +2360,9 @@ int __netlink_dump_start(struct sock *ssk, struct sk_buff *skb,
 	cb->strict_check = !!(nlk2->flags & NETLINK_F_STRICT_CHK);
 
 	if (control->start) {
+		cb->extack = control->extack;
 		ret = control->start(cb);
+		cb->extack = NULL;
 		if (ret)
 			goto error_put;
 	}

net/netlink/genetlink.c

@@ -912,6 +912,7 @@ static int genl_family_rcv_msg_dumpit(const struct genl_family *family,
 			.start = genl_start,
 			.dump = genl_lock_dumpit,
 			.done = genl_lock_done,
+			.extack = extack,
 		};
 
 		genl_unlock();
@@ -924,6 +925,7 @@ static int genl_family_rcv_msg_dumpit(const struct genl_family *family,
 			.start = genl_start,
 			.dump = ops->dumpit,
 			.done = genl_parallel_done,
+			.extack = extack,
 		};
 
 		err = __netlink_dump_start(net->genl_sock, skb, nlh, &c);