netns: no longer hold RTNL in rtnl_net_dumpid()

- rtnl_net_dumpid() is already fully RCU protected,
  RTNL is not needed there.

- Fix return value at the end of a dump,
  so that NLMSG_DONE can be appended to current skb,
  saving one recvmsg() system call.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Link: https://lore.kernel.org/r/20240416140739.967941-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/net_namespace.c

@@ -1090,7 +1090,7 @@ static int rtnl_net_dumpid(struct sk_buff *skb, struct netlink_callback *cb)
 end:
 	if (net_cb.fillargs.add_ref)
 		put_net(net_cb.tgt_net);
-	return err < 0 ? err : skb->len;
+	return err;
 }
 
 static void rtnl_net_notifyid(struct net *net, int cmd, int id, u32 portid,
@@ -1205,7 +1205,8 @@ void __init net_ns_init(void)
 	rtnl_register(PF_UNSPEC, RTM_NEWNSID, rtnl_net_newid, NULL,
 		      RTNL_FLAG_DOIT_UNLOCKED);
 	rtnl_register(PF_UNSPEC, RTM_GETNSID, rtnl_net_getid, rtnl_net_dumpid,
-		      RTNL_FLAG_DOIT_UNLOCKED);
+		      RTNL_FLAG_DOIT_UNLOCKED |
+		      RTNL_FLAG_DUMP_UNLOCKED);
 }
 
 static void free_exit_list(struct pernet_operations *ops, struct list_head *net_exit_list)