target: Drop se_lun->lun_active for existing percpu lun_ref

With se_port_t and t10_alua_tg_pt_gp_member being absored into se_lun,
there is no need for an extra atomic_t based reference count for PR
ALL_TG_PT=1 and ALUA access state transition.

Go ahead and use the existing percpu se_lun->lun_ref instead, and
convert the two special cases to percpu_ref_tryget_live() to avoid
se_lun if transport_clear_lun_ref() has already been invoked to
shutdown the se_lun.

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

include/target/target_core_base.h

@@ -721,7 +721,6 @@ struct se_lun {
 	struct t10_alua_tg_pt_gp *lun_tg_pt_gp;
 	spinlock_t		lun_tg_pt_gp_lock;
 
-	atomic_t		lun_active;
 	struct se_portal_group	*lun_tpg;
 	struct scsi_port_stats	lun_stats;
 	struct config_group	lun_group;