net/ulp: use consistent error code when blocking ULP

The referenced commit changed the error code returned by the kernel
when preventing a non-established socket from attaching the ktls
ULP. Before to such a commit, the user-space got ENOTCONN instead
of EINVAL.

The existing self-tests depend on such error code, and the change
caused a failure:

RUN global.non_established ...
tls.c:1673:non_established:Expected errno (22) == ENOTCONN (107)
non_established: Test failed at step #3
FAIL global.non_established

In the unlikely event existing applications do the same, address
the issue by restoring the prior error code in the above scenario.

Note that the only other ULP performing similar checks at init
time - smc_ulp_ops - also fails with ENOTCONN when trying to attach
the ULP to a non-established socket.

Reported-by: Sabrina Dubroca <[e-mail]>
Fixes: 2c02d41d71f9 ("net/ulp: prevent ULP without clone op from entering the LISTEN status")
Signed-off-by: Paolo Abeni <[e-mail]>
Reviewed-by: Sabrina Dubroca <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/ipv4/tcp_ulp.c

···
 	if (sk->sk_socket)
 		clear_bit(SOCK_SUPPORT_ZC, &sk->sk_socket->flags);
 
-	err = -EINVAL;
+	err = -ENOTCONN;
 	if (!ulp_ops->clone && sk->sk_state == TCP_LISTEN)
 		goto out_err;