xen/mm: Drop the parameter mfn from populate_pt_range

xen/mm: Drop the parameter mfn from populate_pt_range

The function populate_pt_range is used to populate in advance the
page-table but it will not do the actual mapping. So passing the MFN in
parameter is pointless. Note that the only caller pass 0...

At the same time replace 0 by INVALID_MFNs. While this does not matter
as the entry will marked as not valid and populated, INVALID_MFN
helps the reader to know the MFN is invalid.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
--

Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Julien Grall <julien.grall@arm.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Tim Deegan <tim@xen.org>
    Changes in v6:
        - Add George's and Wei's reviewed-by
        - Add Andrew's acked-by

    Changes in v5:
        - Update the commit message to explain why 0 -> INVALID_MFN.

    Changes in v4:
        - Patch added.