arch/tile/mm/init.c: trivial: use BUG_ON

Use BUG_ON(x) rather than if(x) BUG();

The semantic patch that fixes this problem is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@ identifier x; @@
-if (x) BUG();
+BUG_ON(x);

@@ identifier x; @@
-if (!x) BUG();
+BUG_ON(!x);
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>

arch/tile/mm/init.c

@@ -836,8 +836,7 @@ void __init mem_init(void)
 #endif
 
 #ifdef CONFIG_FLATMEM
-	if (!mem_map)
-		BUG();
+	BUG_ON(!mem_map);
 #endif
 
 #ifdef CONFIG_HIGHMEM