[SCSI] esp: Fix OOPS in esp_reset_cleanup().

OOPS reported by Friedrich Oslage <bluebird@porno-bullen.de>

The problem here is that tp->starget is set every time a lun
is allocated for a particular target so we can catch the
sdev_target parent value.

The reset handler uses the NULL'ness of this value to determine
which targets are active.

But esp_slave_destroy() does not NULL out this value when appropriate.

So for every target that doesn't respond, the SCSI bus scan causes
a stale pointer to be left here, with ensuing crashes like you're
seeing.
Signed-off-by: David S. Miller <davem@davemloft.net>
Cc: Stable Tree <stable@kernel.org>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

drivers/scsi/esp_scsi.c

@@ -2371,6 +2371,7 @@ static int esp_slave_alloc(struct scsi_device *dev)
 	dev->hostdata = lp;
 
 	tp->starget = dev->sdev_target;
+	tp->starget_ref++;
 
 	spi_min_period(tp->starget) = esp->min_period;
 	spi_max_offset(tp->starget) = 15;
@@ -2425,10 +2426,17 @@ static int esp_slave_configure(struct scsi_device *dev)
 
 static void esp_slave_destroy(struct scsi_device *dev)
 {
+	struct esp *esp = shost_priv(dev->host);
+	struct esp_target_data *tp = &esp->target[dev->id];
 	struct esp_lun_data *lp = dev->hostdata;
 
 	kfree(lp);
 	dev->hostdata = NULL;
+
+	BUG_ON(tp->starget_ref <= 0);
+
+	if (!--tp->starget_ref)
+		tp->starget = NULL;
 }
 
 static int esp_eh_abort_handler(struct scsi_cmnd *cmd)

drivers/scsi/esp_scsi.h

@@ -322,6 +322,7 @@ struct esp_target_data {
 	u8			nego_goal_tags;
 
 	struct scsi_target	*starget;
+	int			starget_ref;
 };
 
 struct esp_event_ent {