kernel: Constify temporary variable in roundup()

Fix build error with GCC 3.x caused by commit b28efd54
"kernel: roundup should only reference arguments once" by constifying
temporary variable used in that macro.

Signed-off-by: Tetsuo Handa <[e-mail]>
Suggested-by: Andrew Morton <[e-mail]>
Acked-by: Eric Paris <[e-mail]>
Signed-off-by: James Morris <[e-mail]>

include/linux/kernel.h

···
 #define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))
 #define roundup(x, y) (					\
 {							\
-	typeof(y) __y = y;				\
+	const typeof(y) __y = y;			\
 	(((x) + (__y - 1)) / __y) * __y;		\
 }							\
 )