target/configfs: Kill se_lun->lun_link_magic

Instead of using a hardcoded magic value in se_lun when verifying
a target config_item symlink source during target_fabric_mappedlun_link(),
go ahead and use target_fabric_port_item_ops directly instead.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Mike Christie <mchristi@redhat.com>
Cc: Hannes Reinecke <hare@suse.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

include/target/target_core_base.h

@@ -701,8 +701,6 @@ struct scsi_port_stats {
 
 struct se_lun {
 	u64			unpacked_lun;
-#define SE_LUN_LINK_MAGIC			0xffff7771
-	u32			lun_link_magic;
 	bool			lun_shutdown;
 	bool			lun_access_ro;
 	u32			lun_index;