um: Add BUG() to do_ops()'s error path

In case we encounter a bad operation in do_ops() something is really
broken and it's better to BUG().
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/kernel/tlb.c

@@ -75,6 +75,7 @@ static int do_ops(struct host_vm_change *hvc, int end,
 		default:
 			printk(KERN_ERR "Unknown op type %d in do_ops\n",
 			       op->type);
+			BUG();
 			break;
 		}
 	}