mm: provide early_memremap_ro to establish read-only mapping

mm: provide early_memremap_ro to establish read-only mapping

During early boot as Xen pv domain the kernel needs to map some page
tables supplied by the hypervisor read only. This is needed to be
able to relocate some data structures conflicting with the physical
memory map especially on systems with huge RAM (above 512GB).

Provide the function early_memremap_ro() to provide this read only
mapping.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Konrad Rzeszutek Wilk <Konrad.wilk@oracle.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
(cherry picked from commit 2592dbbbf4c67501c2bd2dcf89c2b8924d592a9f)
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>