do_mounts: allow UBI root device name

Similarly to MTD devices, allow UBI devices.
Signed-off-by: NAdrian Hunter <ext-adrian.hunter@nokia.com>

init/do_mounts.c

@@ -372,7 +372,8 @@ void __init prepare_namespace(void)
 
 	if (saved_root_name[0]) {
 		root_device_name = saved_root_name;
-		if (!strncmp(root_device_name, "mtd", 3)) {
+		if (!strncmp(root_device_name, "mtd", 3) ||
+		    !strncmp(root_device_name, "ubi", 3)) {
 			mount_block_root(root_device_name, root_mountflags);
 			goto out;
 		}