arm: mm: fix faulty calculation of area to perform cache operation

[Russell King - ARM Linux]

On Thu, Mar 29, 2012 at 06:55:32PM +0900, Cho KyongHo wrote:
> In dma_cache_maint_page(), if offset is larger than PAGE_SIZE,
> len becomes PAGE_SIZE - offset. If size is smaller than
> PAGE_SIZE - offset, next calcuation of left cause overflow.

It is invalid for offset to be larger than PAGE_SIZE here.