ARM: change vmalloc_start to vmalloc_size

Rather than storing the start of vmalloc space, store the size, and
move the calculation into adjust_lowmem_limit(). We now have one single
place where this calculation takes place.

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>