BLOCK: Fix bounce limit address check

This fixes some OOMs on 64bit systems with <4GB of RAM when accessing
the cdrom.

Do a safer check for when to enable DMA. Currently we enable ISA DMA
for cases that do not need it, resulting in OOM conditions when ZONE_DMA
runs out of space.
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

block/ll_rw_blk.c

@@ -637,7 +637,7 @@ void blk_queue_bounce_limit(request_queue_t *q, u64 dma_addr)
 	/* Assume anything <= 4GB can be handled by IOMMU.
 	   Actually some IOMMUs can handle everything, but I don't
 	   know of a way to test this here. */
-	if (bounce_pfn < (0xffffffff>>PAGE_SHIFT))
+	if (bounce_pfn < (min_t(u64,0xffffffff,BLK_BOUNCE_HIGH) >> PAGE_SHIFT))
 		dma = 1;
 	q->bounce_pfn = max_low_pfn;
 #else