[S390] dcssblk: fix race in dcssblk_add_store()

Concurrently adding the same segment may lead to duplicate device_register()
calls, which will trigger an error in kobject code:
"... don't try to register things with the same name in the same directory".
This patch adds a check to avoid this.
Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/block/dcssblk.c

@@ -384,6 +384,10 @@ dcssblk_add_store(struct device *dev, struct device_attribute *attr, const char
 	 * get minor, add to list
 	 */
 	down_write(&dcssblk_devices_sem);
+	if (dcssblk_get_segment_by_name(local_buf)) {
+		rc = -EEXIST;
+		goto release_gd;
+	}
 	rc = dcssblk_assign_free_minor(dev_info);
 	if (rc) {
 		up_write(&dcssblk_devices_sem);