From: Rusty Russell <rusty@rustcorp.com.au>

copy_device_tree calls inspect_node() which recurses to copy the Open Firmware
device tree.  It just copied into memory above the kernel for about 8MB, which
can overwrite the initrd, and/or get truncated on big machines.

The real solution is to allocate memory properly for all the prom_init memory
users.  However, that's quite a change.  This patch introduces a "make_room"
macro which inspect_node uses to get memory.  This does the same as before,
except skips over the initrd if neccessary.