spapr: Improve handling of fdt buffer size

spapr: Improve handling of fdt buffer size

Previously, spapr_build_fdt() constructed the device tree in a fixed
buffer of size FDT_MAX_SIZE.  This is a bit inflexible, but more
importantly it's awkward for the case where we use it during CAS.  In
that case the guest firmware supplies a buffer and we have to
awkwardly check that what we generated fits into it afterwards, after
doing a lot of size checks during spapr_build_fdt().

Simplify this by having spapr_build_fdt() take a 'space' parameter.
For the CAS case, we pass in the buffer size provided by SLOF, for the
machine init case, we continue to pass FDT_MAX_SIZE.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Cedric Le Goater <clg@fr.ibm.com>
Reviewed-by: Greg Kurz <groug@kaod.org>