l2tp: avoid overriding sk->sk_user_data

Although commit 4a4cd703 ("l2tp: don't set sk_user_data in tunnel socket")
removed sk->sk_user_data usage, setup_udp_tunnel_sock() still touches
sk->sk_user_data, this conflicts with sockmap which also leverages
sk->sk_user_data to save psock.

Restore this sk->sk_user_data check to avoid such conflicts.

Fixes: 4a4cd703 ("l2tp: don't set sk_user_data in tunnel socket")
Reported-by:  <syzbot+8dbe3133b840c470da0e@syzkaller.appspotmail.com>
Cc: Tom Parkin <tparkin@katalix.com>
Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Tested-by: James Chapman <jchapman@katalix.com>
Reviewed-by: James Chapman <jchapman@katalix.com>
Link: https://patch.msgid.link/20240822182544.378169-1-xiyou.wangcong@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>