target: Add link_magic for fabric allow_link destination target_items

This patch adds [dev,lun]_link_magic value assignment + checks within generic
target_fabric_port_link() and target_fabric_mappedlun_link() code to ensure
destination config_item *target_item sent from configfs_symlink() ->
config_item_operations->allow_link() is the underlying se_device->dev_group
and se_lun->lun_group that we expect to symlink.

Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: stable@vger.kernel.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

include/target/target_core_base.h

@@ -624,6 +624,8 @@ struct se_dev_stat_grps {
 };
 
 struct se_device {
+#define SE_DEV_LINK_MAGIC			0xfeeddeef
+	u32			dev_link_magic;
 	/* RELATIVE TARGET PORT IDENTIFER Counter */
 	u16			dev_rpti_counter;
 	/* Used for SAM Task Attribute ordering */
@@ -722,6 +724,8 @@ struct se_port_stat_grps {
 };
 
 struct se_lun {
+#define SE_LUN_LINK_MAGIC			0xffff7771
+	u32			lun_link_magic;
 	/* See transport_lun_status_table */
 	enum transport_lun_status_table lun_status;
 	u32			lun_access;