qcacld-3.0: Add flush callback for umac stop

During the scheduler_stop it tries to flush all the pending messages
in the queue, during the same the flush callbacks of all the pending
messages are invoked. If there is no flush callback for
SYS_MSG_ID_UMAC_STOP, it will free the bodyptr of the message which
contains the global variable g_stop_evt, which can result in invalid
access in the other thread.

Add a dummy flush callback message for the sys_stop message to
avoid the invalid access.

Change-Id: I46ca4240d787dce5b21cc2f99ced66a4c01b737c
CRs-Fixed: 2537773

core/mac/src/sys/common/src/wlan_qct_sys.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2012-2019 The Linux Foundation. All rights reserved.
+ * Copyright (c) 2012-2020 The Linux Foundation. All rights reserved.
  *
  * Permission to use, copy, modify, and/or distribute this software for
  * any purpose with or without fee is hereby granted, provided that the
@@ -72,6 +72,11 @@ static void umac_stop_complete_cb(void *user_data)
 }
 #endif
 
+static inline QDF_STATUS umac_stop_flush_cb(struct scheduler_msg *msg)
+{
+	return QDF_STATUS_SUCCESS;
+}
+
 /**
  * umac_stop() - To post stop message to system module
  *
@@ -96,6 +101,7 @@ QDF_STATUS umac_stop(void)
 	/* Save the user callback and user data */
 	umac_stop_msg.callback = umac_stop_complete_cb;
 	umac_stop_msg.bodyptr = (void *)&g_stop_evt;
+	umac_stop_msg.flush_callback = umac_stop_flush_cb;
 
 	/* post the message.. */
 	qdf_status = scheduler_post_message(QDF_MODULE_ID_SYS,