target: kill struct se_subsystem_dev

Simplify the code a lot by killing the superflous struct se_subsystem_dev.
Instead se_device is allocated early on by the backend driver, which allocates
it as part of its own per-device structure, borrowing the scheme that is for
example used for inode allocation.

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

drivers/target/loopback/tcm_loop.h

@@ -53,7 +53,6 @@ struct tcm_loop_hba {
 	struct se_hba_s *se_hba;
 	struct se_lun *tl_hba_lun;
 	struct se_port *tl_hba_lun_sep;
-	struct se_device_s *se_dev_hba_ptr;
 	struct tcm_loop_nexus *tl_nexus;
 	struct device dev;
 	struct Scsi_Host *sh;