mptcp: fix tracking issue in mptcp_subflow_create_socket()

My recent patch missed that mptcp_subflow_create_socket()
was creating a 'kernel' socket, then converted it to 'user' socket.

Fixes: 0cafd77 ("net: add a refcount tracker for kernel sockets")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Matthieu Baerts <matthieu.baerts@tessares.net>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Link: https://lore.kernel.org/r/20221025180546.652251-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/subflow.c

@@ -1595,7 +1595,9 @@ int mptcp_subflow_create_socket(struct sock *sk, struct socket **new_sock)
 
 	/* kernel sockets do not by default acquire net ref, but TCP timer
 	 * needs it.
+	 * Update ns_tracker to current stack trace and refcounted tracker.
 	 */
+	__netns_tracker_free(net, &sf->sk->ns_tracker, false);
 	sf->sk->sk_net_refcnt = 1;
 	get_net_track(net, &sf->sk->ns_tracker, GFP_KERNEL);
 	sock_inuse_add(net, 1);