x86/mm: Combine create_grant_{pte,va}_mapping()

x86/mm: Combine create_grant_{pte,va}_mapping()

create_grant_{pte,va}_mapping() are nearly identical; all that is really
different between them is how they convert their addr parameter to the pte to
install the grant into.

Reimplement their logic in create_grant_pv_mapping() in a mostly common way.

No (intended) change in behaviour from a guests point of view.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>