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 <penguin-kernel@I-love.SAKURA.ne.jp>
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>