ARM: change vmalloc_min to be unsigned long

vmalloc_min is currently a void pointer, but everywhere its used
contains a cast - either to a void pointer when setting or back to
an integer type when being used. Eliminate these casts by changing
its type to unsigned long.

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>