arm64: pgd: Mark pgd_cache as __ro_after_init

pgd_cache is setup once while init stage and never changed after
that, so it is good candidate for __ro_after_init

Signed-off-by: Jinbum Park <jinb.park7@gmail.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/mm/pgd.c

@@ -26,7 +26,7 @@
 #include <asm/page.h>
 #include <asm/tlbflush.h>
 
-static struct kmem_cache *pgd_cache;
+static struct kmem_cache *pgd_cache __ro_after_init;
 
 pgd_t *pgd_alloc(struct mm_struct *mm)
 {