[ARM] prevent the vmalloc cmdline argument from eating all memory

Commit 8d5796d2 allows for the vmalloc
area to be resized from the kernel cmdline.  Make sure it cannot overlap
with RAM entirely.

Signed-off-by: Nicolas Pitre <nico@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>