tcp, ulp: fix leftover icsk_ulp_ops preventing sock from reattach

I found that in BPF sockmap programs once we either delete a socket
from the map or we updated a map slot and the old socket was purged
from the map that these socket can never get reattached into a map
even though their related psock has been dropped entirely at that
point.

Reason is that tcp_cleanup_ulp() leaves the old icsk->icsk_ulp_ops
intact, so that on the next tcp_set_ulp_id() the kernel returns an
-EEXIST thinking there is still some active ULP attached.

BPF sockmap is the only one that has this issue as the other user,
kTLS, only calls tcp_cleanup_ulp() from tcp_v4_destroy_sock() whereas
sockmap semantics allow dropping the socket from the map with all
related psock state being cleaned up.

Fixes: 1aa12bdf1b ("bpf: sockmap, add sock close() hook to remove socks")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

net/ipv4/tcp_ulp.c

@@ -129,6 +129,8 @@ void tcp_cleanup_ulp(struct sock *sk)
 	if (icsk->icsk_ulp_ops->release)
 		icsk->icsk_ulp_ops->release(sk);
 	module_put(icsk->icsk_ulp_ops->owner);
+
+	icsk->icsk_ulp_ops = NULL;
 }
 
 /* Change upper layer protocol for socket */