ARM: use a temporary variable to hold maximum vmalloc size

We calculate the maximum size of the vmalloc space twice in
early_vmalloc(). Use a temporary variable to hold this value.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Yanfei Xu <yanfei.xu@windriver.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>