scsi: libiscsi: Fix UAF in iscsi_conn_get_param()/iscsi_conn_teardown()

|- iscsi_if_destroy_conn |-dev_attr_show
|-iscsi_conn_teardown
|-spin_lock_bh |-iscsi_sw_tcp_conn_get_param

|-kfree(conn->persistent_address) |-iscsi_conn_get_param
|-kfree(conn->local_ipaddr)
==>|-read persistent_address
==>|-read local_ipaddr
|-spin_unlock_bh

When iscsi_conn_teardown() and iscsi_conn_get_param() happen in parallel, a
UAF may be triggered.

Link: https://lore.kernel.org/r/[e-mail]
Reported-by: Lu Tixiong <[e-mail]>
Reviewed-by: Mike Christie <[e-mail]>
Reviewed-by: Lee Duncan <[e-mail]>
Signed-off-by: Lixiaokeng <[e-mail]>
Signed-off-by: Linfeilong <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/scsi/libiscsi.c

···
 {
 	struct iscsi_conn *conn = cls_conn->dd_data;
 	struct iscsi_session *session = conn->session;
+	char *tmp_persistent_address = conn->persistent_address;
+	char *tmp_local_ipaddr = conn->local_ipaddr;
 
 	del_timer_sync(&conn->transport_timer);
 
···
 	spin_lock_bh(&session->frwd_lock);
 	free_pages((unsigned long) conn->data,
 		   get_order(ISCSI_DEF_MAX_RECV_SEG_LEN));
-	kfree(conn->persistent_address);
-	kfree(conn->local_ipaddr);
 	/* regular RX path uses back_lock */
 	spin_lock_bh(&session->back_lock);
 	kfifo_in(&session->cmdpool.queue, (void*)&conn->login_task,
···
 	mutex_unlock(&session->eh_mutex);
 
 	iscsi_destroy_conn(cls_conn);
+	kfree(tmp_persistent_address);
+	kfree(tmp_local_ipaddr);
 }
 EXPORT_SYMBOL_GPL(iscsi_conn_teardown);