xen/x86: streamline set_pte_mfn()

xen/x86: streamline set_pte_mfn()

In preparation for restoring xen_set_pte_init()'s original behavior of
avoiding hypercalls, make set_pte_mfn() no longer use the standard
set_pte() code path. That one is more complicated than the alternative
of simply using an available hypercall directly. This way we can avoid
introducing a fair number (2k on my test system) of cases where the
hypervisor would trap-and-emulate page table updates.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/b39c08e8-4a53-8bca-e6e7-3684a6cab8d0@suse.com
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>