x86/mm: introduce modify_xen_mappings()

x86/mm: introduce modify_xen_mappings()

To simply change the permissions on existing Xen mappings.  The existing
destroy_xen_mappings() is altered to support changing the PTE permissions.

A new destroy_xen_mappings() is introduced, as the special case of not passing
_PAGE_PRESENT to modify_xen_mappings().

As cleanup (and an ideal functional test), the boot logic which remaps Xen's
code and data with reduced permissions is altered to use
modify_xen_mappings(), rather than map_pages_to_xen() passing the same mfn's
back in.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>