iscsi-target: Fix early sk_data_ready LOGIN_FLAGS_READY race

There is a iscsi-target/tcp login race in LOGIN_FLAGS_READY
state assignment that can result in frequent errors during
iscsi discovery:

      "iSCSI Login negotiation failed."

To address this bug, move the initial LOGIN_FLAGS_READY
assignment ahead of iscsi_target_do_login() when handling
the initial iscsi_target_start_negotiation() request PDU
during connection login.

As iscsi_target_do_login_rx() work_struct callback is
clearing LOGIN_FLAGS_READ_ACTIVE after subsequent calls
to iscsi_target_do_login(), the early sk_data_ready
ahead of the first iscsi_target_do_login() expects
LOGIN_FLAGS_READY to also be set for the initial
login request PDU.

As reported by Maged, this was first obsered using an
MSFT initiator running across multiple VMWare host
virtual machines with iscsi-target/tcp.
Reported-by: Maged Mokhtar <mmokhtar@binarykinetics.com>
Tested-by: Maged Mokhtar <mmokhtar@binarykinetics.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/iscsi_target_nego.c

@@ -1247,8 +1247,6 @@ int iscsi_target_start_negotiation(
 {
 	int ret;
 
-	ret = iscsi_target_do_login(conn, login);
-	if (!ret) {
        if (conn->sock) {
                struct sock *sk = conn->sock->sk;
 
@@ -1256,7 +1254,9 @@ int iscsi_target_start_negotiation(
                set_bit(LOGIN_FLAGS_READY, &conn->login_flags);
                write_unlock_bh(&sk->sk_callback_lock);
        }
-	} else if (ret < 0) {
+
+       ret = iscsi_target_do_login(conn, login);
+       if (ret < 0) {
 		cancel_delayed_work_sync(&conn->login_work);
 		cancel_delayed_work_sync(&conn->login_cleanup_work);
 		iscsi_target_restore_sock_callbacks(conn);