netpoll: Use rtnl_dereference() for npinfo pointer access

In the __netpoll_setup() function, when accessing the device's npinfo
pointer, replace rcu_access_pointer() with rtnl_dereference(). This
change is more appropriate, as suggested by Herbert Xu[1].

The function is called with the RTNL mutex held, and the pointer is
being dereferenced later, so, dereference earlier and just reuse the
pointer for the if/else.

The replacement ensures correct pointer access while maintaining
the existing locking and RCU semantics of the netpoll subsystem.

Link: https://lore.kernel.org/lkml/Zz1cKZYt1e7elibV@gondor.apana.org.au/ [1]
Suggested-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Link: https://patch.msgid.link/20241202-netpoll_rcu_herbet_fix-v2-1-2b9d58edc76a@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/netpoll.c

@@ -634,7 +634,8 @@ int __netpoll_setup(struct netpoll *np, struct net_device *ndev)
 		goto out;
 	}
 
-	if (!rcu_access_pointer(ndev->npinfo)) {
+	npinfo = rtnl_dereference(ndev->npinfo);
+	if (!npinfo) {
 		npinfo = kmalloc(sizeof(*npinfo), GFP_KERNEL);
 		if (!npinfo) {
 			err = -ENOMEM;
@@ -654,7 +655,6 @@ int __netpoll_setup(struct netpoll *np, struct net_device *ndev)
 				goto free_npinfo;
 		}
 	} else {
-		npinfo = rtnl_dereference(ndev->npinfo);
 		refcount_inc(&npinfo->refcnt);
 	}