ARM: 8734/1: mm: idmap: Mark variables as ro_after_init

idmap_pgd, arch_phys_to_idmap_offset are setup once
while init stage, and never changed after that.
so, it is good candidate for __ro_after_init.
Reviewed-by: NKees Cook <keescook@chromium.org>
Signed-off-by: NJinbum Park <jinb.park7@gmail.com>
Signed-off-by: NRussell King <rmk+kernel@armlinux.org.uk>

arch/arm/mm/idmap.c

@@ -16,8 +16,8 @@
  * are not supported on any CPU using the idmap tables as its current
  * page tables.
  */
-pgd_t *idmap_pgd;
-long long arch_phys_to_idmap_offset;
+pgd_t *idmap_pgd __ro_after_init;
+long long arch_phys_to_idmap_offset __ro_after_init;
 
 #ifdef CONFIG_ARM_LPAE
 static void idmap_add_pmd(pud_t *pud, unsigned long addr, unsigned long end,