spapr,ppc: Simplify signature of kvmppc_rma_size()

spapr,ppc: Simplify signature of kvmppc_rma_size()

This function calculates the maximum size of the RMA as implied by the
host's page size of structure of the VRMA (there are a number of other
constraints on the RMA size which will supersede this one in many
circumstances).

The current interface takes the current RMA size estimate, and clamps it
to the VRMA derived size.  The only current caller passes in an arguably
wrong value (it will match the current RMA estimate in some but not all
cases).

We want to fix that, but for now just keep concerns separated by having the
KVM helper function just return the VRMA derived limit, and let the caller
combine it with other constraints.  We call the new function
kvmppc_vrma_limit() to more clearly indicate its limited responsibility.

The helper should only ever be called in the KVM enabled case, so replace
its !CONFIG_KVM stub with an assert() rather than a dummy value.

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>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>