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>