xen/x86: adjust xen_set_fixmap()

Using __native_set_fixmap() here means guaranteed trap-and-emulate
instances the hypervisor has to deal with. Since the virtual address
covered by the to be adjusted page table entry is easy to determine (and
actually already gets obtained in a special case), simply use an
available, easy to invoke hypercall instead.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/11fcaea2-ec17-3edd-ecdf-4cdd2d472bd0@suse.com


Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>