The problem was that IDE-disks on machines with IDE harddisks,
memory above 4GB and no hardware I/O TLB would go BUG_ON() in
blk_queue_bounce_limit() because the IDE-controller could only
address 4GB of memory and that was much less than BLK_BOUNCE_ISA
(which is equal to ISA_DMA_THRESHOLD).  Note that the problem does
NOT trigger with CD-ROMs, which always uses the software I/O TLB
(and hence bounce-buffers) on such machines.

The best fix seems to be to simply lower ISA_DMA_THRESHOLD to 4GB-1 since
that will ensure that the IDE block layer allocates memory with GFP_DMA,
which will minimize bounce buffering.