powerpc/kvm/cma: Fix panic introduces by signed shift operation

fc95ca72 introduces a memset in
kvmppc_alloc_hpt since the general CMA doesn't clear the memory it
allocates.

However, the size argument passed to memset is computed from a signed value
and its signed bit is extended by the cast the compiler is doing. This lead
to extremely large size value when dealing with order value >= 31, and
almost all the memory following the allocated space is cleaned. As a
consequence, the system is panicing and may even fail spawning the kdump
kernel.

This fix makes use of an unsigned value for the memset's size argument to
avoid sign extension. Among this fix, another shift operation which may
lead to signed extended value too is also fixed.

Cc: Alexey Kardashevskiy <aik@ozlabs.ru>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Alexander Graf <agraf@suse.de>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>