ARM: dma-api: fix off-by-one error in __dma_supported()

When validating the mask against the amount of memory we have available
(so that we can trap 32-bit DMA addresses with >32-bits memory), we had
not taken account of the fact that max_pfn is the maximum PFN number
plus one that would be in the system.

There are several references in the code which bear this out:

mm/page_owner.c:
	for (; pfn < max_pfn; pfn++) {
	}

arch/x86/kernel/setup.c:
	high_memory = (void *)__va(max_pfn * PAGE_SIZE - 1)
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/mm/dma-mapping.c

@@ -171,7 +171,7 @@ static int __dma_supported(struct device *dev, u64 mask, bool warn)
 	 */
 	if (sizeof(mask) != sizeof(dma_addr_t) &&
 	    mask > (dma_addr_t)~0 &&
-	    dma_to_pfn(dev, ~0) < max_pfn) {
+	    dma_to_pfn(dev, ~0) < max_pfn - 1) {
 		if (warn) {
 			dev_warn(dev, "Coherent DMA mask %#llx is larger than dma_addr_t allows\n",
 				 mask);