[POWERPC] Fix large hash table allocation on Cell blades

My recent hack to allocate the hash table under 1GB on cell was poorly
tested, *cough*. It turns out on blades with large amounts of memory we
fail to allocate the hash table at all. This is because RTAS has been
instantiated just below 768MB, and 0-x MB are used by the kernel,
leaving no areas that are both large enough and also naturally-aligned.

For the cell IOMMU hack the page tables must be under 2GB, so use that
as the limit instead. This has been tested on real hardware and boots
happily.
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/mm/hash_utils_64.c

@@ -506,10 +506,10 @@ void __init htab_initialize(void)
 	} else {
 		/* Find storage for the HPT.  Must be contiguous in
 		 * the absolute address space. On cell we want it to be
-		 * in the first 1 Gig.
+		 * in the first 2 Gig so we can use it for IOMMU hacks.
 		 */
 		if (machine_is(cell))
-			limit = 0x40000000;
+			limit = 0x80000000;
 		else
 			limit = 0;