ia64: Fix IDE block-layer BUG_ON() reported by Darren Williams.

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.

include/asm-ia64/scatterlist.h

@@ -2,7 +2,7 @@
 #define _ASM_IA64_SCATTERLIST_H
 
 /*
- * Modified 1998-1999, 2001-2002
+ * Modified 1998-1999, 2001-2002, 2004
  *	David Mosberger-Tang <davidm@hpl.hp.com>, Hewlett-Packard Co
  */
 
@@ -15,6 +15,14 @@ struct scatterlist {
 	unsigned int dma_length;
 };
 
-#define ISA_DMA_THRESHOLD	(~0UL)
+/*
+ * It used to be that ISA_DMA_THRESHOLD had something to do with the
+ * DMA-limits of ISA-devices.  Nowadays, its only remaining use (apart
+ * from the aha1542.c driver, which isn't 64-bit clean anyhow) is to
+ * tell the block-layer (via BLK_BOUNCE_ISA) what the max. physical
+ * address of a page is that is allocated with GFP_DMA.  On IA-64,
+ * that's 4GB - 1.
+ */
+#define ISA_DMA_THRESHOLD	0xffffffff
 
 #endif /* _ASM_IA64_SCATTERLIST_H */