iscsi-target: Fix incorrect np->np_thread NULL assignment

When shutting down a target there is a race condition between
iscsit_del_np() and __iscsi_target_login_thread().
The latter sets the thread pointer to NULL, and the former
tries to issue kthread_stop() on that pointer without any
synchronization.

This patch moves the np->np_thread NULL assignment into
iscsit_del_np(), after kthread_stop() has completed. It also
removes the signal_pending() + np_state check, and only
exits when kthread_should_stop() is true.

Reported-by: Hannes Reinecke <hare@suse.de>
Cc: <stable@vger.kernel.org> #3.12+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/iscsi_target.c

@@ -465,6 +465,7 @@ int iscsit_del_np(struct iscsi_np *np)
 		 */
 		send_sig(SIGINT, np->np_thread, 1);
 		kthread_stop(np->np_thread);
+		np->np_thread = NULL;
 	}
 
 	np->np_transport->iscsit_free_np(np);

drivers/target/iscsi/iscsi_target_login.c

@@ -1403,19 +1403,13 @@ static int __iscsi_target_login_thread(struct iscsi_np *np)
 
 out:
 	stop = kthread_should_stop();
-	if (!stop && signal_pending(current)) {
-		spin_lock_bh(&np->np_thread_lock);
-		stop = (np->np_thread_state == ISCSI_NP_THREAD_SHUTDOWN);
-		spin_unlock_bh(&np->np_thread_lock);
-	}
 	/* Wait for another socket.. */
 	if (!stop)
 		return 1;
 exit:
 	iscsi_stop_login_thread_timer(np);
 	spin_lock_bh(&np->np_thread_lock);
 	np->np_thread_state = ISCSI_NP_THREAD_EXIT;
-	np->np_thread = NULL;
 	spin_unlock_bh(&np->np_thread_lock);
 
 	return 0;