neighbour: use RCU protection in __neigh_notify()

JIRA: https://issues.redhat.com/browse/RHEL-87474
Conflicts: \
  - different context, as rhel-9 lacks the upstream \
  commit c25bdd2 ("neighbour: delete redundant judgment statements") \
  - use dev_net() instead of dev_net_rcu() as rhel-9 lacks the upstream \
  commit 482ad2a ("net: add dev_net_rcu() helper")

Upstream commit:
commit becbd58
Author: Eric Dumazet <edumazet@google.com>
Date:   Fri Feb 7 13:58:35 2025 +0000

    neighbour: use RCU protection in __neigh_notify()

    __neigh_notify() can be called without RTNL or RCU protection.

    Use RCU protection to avoid potential UAF.

    Fixes: 426b530 ("[NETNS]: Modify the neighbour table code so it handles multiple network namespaces")
    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Reviewed-by: David Ahern <dsahern@kernel.org>
    Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
    Link: https://patch.msgid.link/20250207135841.1948589-4-edumazet@google.com
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: Paolo Abeni

net/core/neighbour.c

@@ -3469,10 +3469,12 @@ static const struct seq_operations neigh_stat_seq_ops = {
 static void __neigh_notify(struct neighbour *n, int type, int flags,
 			   u32 pid)
 {
-	struct net *net = dev_net(n->dev);
 	struct sk_buff *skb;
 	int err = -ENOBUFS;
+	struct net *net;
 
+	rcu_read_lock();
+	net = dev_net(n->dev);
 	skb = nlmsg_new(neigh_nlmsg_size(), GFP_ATOMIC);
 	if (skb == NULL)
 		goto errout;
@@ -3485,10 +3487,12 @@ static void __neigh_notify(struct neighbour *n, int type, int flags,
 		goto errout;
 	}
 	rtnl_notify(skb, net, 0, RTNLGRP_NEIGH, NULL, GFP_ATOMIC);
-	return;
+	goto out;
 errout:
 	if (err < 0)
 		rtnl_set_sk_err(net, RTNLGRP_NEIGH, err);
+out:
+	rcu_read_unlock();
 }
 
 void neigh_app_ns(struct neighbour *n)