iscsi-target: make sure to wake up sleeping login worker

Mike Christie reports:
Starting in 4.14 iscsi logins will fail around 50% of the time.

Problem appears to be that iscsi_target_sk_data_ready() callback may
return without doing anything in case it finds the login work queue
is still blocked in sock_recvmsg().

Nicholas Bellinger says:
It would indicate users providing their own ->sk_data_ready() callback
must be responsible for waking up a kthread context blocked on
sock_recvmsg(..., MSG_WAITALL), when a second ->sk_data_ready() is
received before the first sock_recvmsg(..., MSG_WAITALL) completes.

So, do this and invoke the original data_ready() callback -- in
case of tcp sockets this takes care of waking the thread.

Disclaimer: I do not understand why this problem did not show up before
tcp prequeue removal.

(Drop WARN_ON usage - nab)

Reported-by: Mike Christie <[e-mail]>
Bisected-by: Mike Christie <[e-mail]>
Tested-by: Mike Christie <[e-mail]>
Diagnosed-by: Nicholas Bellinger <[e-mail]>
Fixes: e7942d0633c4 ("tcp: remove prequeue support")
Signed-off-by: Florian Westphal <[e-mail]>
Cc: [e-mail] # 4.14+
Signed-off-by: Nicholas Bellinger <[e-mail]>

drivers/target/iscsi/iscsi_target_nego.c

···
 	if (test_and_set_bit(LOGIN_FLAGS_READ_ACTIVE, &conn->login_flags)) {
 		write_unlock_bh(&sk->sk_callback_lock);
 		pr_debug("Got LOGIN_FLAGS_READ_ACTIVE=1, conn: %p >>>>\n", conn);
+		if (iscsi_target_sk_data_ready == conn->orig_data_ready)
+			return;
+		conn->orig_data_ready(sk);
 		return;
 	}