[SCSI] zfcp: return early from slave_destroy if slave_alloc returned early

zfcp_scsi_slave_destroy erroneously always tried to finish its task
even if the corresponding previous zfcp_scsi_slave_alloc returned
early. This can lead to kernel page faults on accessing uninitialized
fields of struct zfcp_scsi_dev in zfcp_erp_lun_shutdown_wait. Take the
port field of the struct to determine if slave_alloc returned early.

This zfcp bug is exposed by 4e6c82b3 (in turn fixing f7c9c6bb to be
compatible with 21208ae5) which can call slave_destroy for a
corresponding previous slave_alloc that did not finish.

This patch is based on James Bottomley's fix suggestion in
http://www.spinics.net/lists/linux-scsi/msg55449.html.
Signed-off-by: Steffen Maier <maier@linux.vnet.ibm.com>
Cc: <stable@kernel.org> #2.6.38+
Signed-off-by: James Bottomley <JBottomley@Parallels.com>

drivers/s390/scsi/zfcp_scsi.c

@@ -55,6 +55,10 @@ static void zfcp_scsi_slave_destroy(struct scsi_device *sdev)
 {
 	struct zfcp_scsi_dev *zfcp_sdev = sdev_to_zfcp(sdev);
 
+	/* if previous slave_alloc returned early, there is nothing to do */
+	if (!zfcp_sdev->port)
+		return;
+
 	zfcp_erp_lun_shutdown_wait(sdev, "scssd_1");
 	put_device(&zfcp_sdev->port->dev);
 }