diag: Protect the decrement of number of diag clients

In diagchar_open() protect the decrement of number of diag clients
so that there will be no race conditions while reading the value
from other functions.

Change-Id: I0e2fb5331eec9c7bba39e7d881b69559256833a3
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>
diff --git a/drivers/char/diag/diagchar_core.c b/drivers/char/diag/diagchar_core.c
index 0158549..1e0a026 100644
--- a/drivers/char/diag/diagchar_core.c
+++ b/drivers/char/diag/diagchar_core.c
@@ -1,4 +1,4 @@
-/* Copyright (c) 2008-2017, The Linux Foundation. All rights reserved.
+/* Copyright (c) 2008-2018, The Linux Foundation. All rights reserved.
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 2 and
@@ -374,8 +374,8 @@
 	return -ENOMEM;
 
 fail:
-	mutex_unlock(&driver->diagchar_mutex);
 	driver->num_clients--;
+	mutex_unlock(&driver->diagchar_mutex);
 	pr_err_ratelimited("diag: Insufficient memory for new client");
 	return -ENOMEM;
 }