target: Fix left-over se_lun->lun_sep pointer OOPs

This patch fixes a left-over se_lun->lun_sep pointer OOPs when one
of the /sys/kernel/config/target/$FABRIC/$WWPN/$TPGT/lun/$LUN/alua*
attributes is accessed after the $DEVICE symlink has been removed.

To address this bug, go ahead and clear se_lun->lun_sep memory in
core_dev_unexport(), so that the existing checks for show/store
ALUA attributes in target_core_fabric_configfs.c work as expected.

Reported-by: Sebastian Herbszt <[e-mail]>
Tested-by: Sebastian Herbszt <[e-mail]>
Cc: [e-mail]
Signed-off-by: Nicholas Bellinger <[e-mail]>

drivers/target/target_core_device.c

···
 	dev->export_count--;
 	spin_unlock(&hba->device_lock);
 
+	lun->lun_sep = NULL;
 	lun->lun_se_dev = NULL;
 }