spapr: sanity check size of the CAS buffer

The CAS buffer is provided by SLOF. A broken SLOF could pass a silly
size: either smaller than the diff header, in which case the current
code will try to allocate 16 Exabytes of memory and g_malloc0() will
abort, or bigger than the maximum memory provisioned for SLOF (ie,
40 Megabytes), which doesn't make sense. Both cases indicate that
SLOF has a bug.

Let's print out an explicit error message and exit since rebooting as
we do with other errors would only result in a reset loop.

Signed-off-by: Greg Kurz <groug@kaod.org>
[dwg: Fix format specifier that broke 32-bit builds]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
