[prefix] Limit initrd to the low 2GB of memory

When using x86 Physical Address Extensions (PAE), winload.exe seems to
assume (in winload!MmPaeInitializeTranslation()) that all memory from
0x80000000 upwards is available for allocation, regardless of the
contents of the BOOTAPP structure memory region descriptors.  This
leads to odd failures when winload.exe attempts to subsequently create
memory map entries for pages that have already been mapped as part of
the BOOTAPP-described region.

Work around this assumption by forcing the initrd below 0x80000000
(via the ramdisk_max field in the bzImage header).

Reported-by: Casper <thephantom1492@yahoo.com>
Debugged-by: Christian Nilsson <nikize@gmail.com>
Debugged-by: Michael Brown <mbrown@fensystems.co.uk>
Tested-by: Casper <thephantom1492@yahoo.com>
Tested-by: Christian Nilsson <nikize@gmail.com>
Signed-off-by: Michael Brown <mbrown@fensystems.co.uk>

src/prefix.S

@@ -243,7 +243,7 @@ cmd_line_ptr:
 
 	.org	0x22c
 ramdisk_max:
-	.long	0xffffffff
+	.long	0x7fffffff
 
 version_string:
 	.asciz	VERSION