target: Fix possible NULL pointer with __transport_execute_tasks

This patch makes __transport_execute_tasks() use a local *se_dev
reference to prevent direct se_cmd->se_dev access after
transport_cmd_check_stop() -> transport_add_tasks_from_cmd()
has been called, as in the current implementation we can expect
__transport_execute_tasks() may be called from another context
that may have already completed the I/O.

Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_transport.c

@@ -2076,6 +2076,7 @@ static inline int transport_execute_task_attr(struct se_cmd *cmd)
 static int transport_execute_tasks(struct se_cmd *cmd)
 {
 	int add_tasks;
+	struct se_device *se_dev = cmd->se_dev;
 
 	if (se_dev_check_online(cmd->se_dev) != 0) {
 		cmd->scsi_sense_reason = TCM_LOGICAL_UNIT_COMMUNICATION_FAILURE;
@@ -2108,7 +2109,7 @@ static int transport_execute_tasks(struct se_cmd *cmd)
 	 * storage object.
 	 */
 execute_tasks:
-	__transport_execute_tasks(cmd->se_dev);
+	__transport_execute_tasks(se_dev);
 	return 0;
 }