arm64: mm: move pgd_cache initialisation to pgtable_cache_init

Initialising the suppport for EFI runtime services requires us to
allocate a pgd off the back of an early_initcall. On systems where the
PGD_SIZE is smaller than PAGE_SIZE (e.g. 64k pages and 48-bit VA), the
pgd_cache isn't initialised at this stage, and we panic with a NULL
dereference during boot:

  Unable to handle kernel NULL pointer dereference at virtual address 00000000

  __create_mapping.isra.5+0x84/0x350
  create_pgd_mapping+0x20/0x28
  efi_create_mapping+0x5c/0x6c
  arm_enable_runtime_services+0x154/0x1e4
  do_one_initcall+0x8c/0x190
  kernel_init_freeable+0x84/0x1ec
  kernel_init+0x10/0xe0
  ret_from_fork+0x10/0x50

This patch fixes the problem by initialising the pgd_cache earlier, in
the pgtable_cache_init callback, which sounds suspiciously like what it
was intended for.
Reported-by: Dennis Chen <dennis.chen@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/pgtable.h

@@ -676,7 +676,8 @@ extern int kern_addr_valid(unsigned long addr);
 
 #include <asm-generic/pgtable.h>
 
-#define pgtable_cache_init() do { } while (0)
+void pgd_cache_init(void);
+#define pgtable_cache_init	pgd_cache_init
 
 /*
  * On AArch64, the cache coherency is handled via the set_pte_at() function.

arch/arm64/mm/pgd.c

@@ -46,14 +46,14 @@ void pgd_free(struct mm_struct *mm, pgd_t *pgd)
 		kmem_cache_free(pgd_cache, pgd);
 }
 
-static int __init pgd_cache_init(void)
+void __init pgd_cache_init(void)
 {
+	if (PGD_SIZE == PAGE_SIZE)
+		return;
+
 	/*
 	 * Naturally aligned pgds required by the architecture.
 	 */
-	if (PGD_SIZE != PAGE_SIZE)
-		pgd_cache = kmem_cache_create("pgd_cache", PGD_SIZE, PGD_SIZE,
-					      SLAB_PANIC, NULL);
-	return 0;
+	pgd_cache = kmem_cache_create("pgd_cache", PGD_SIZE, PGD_SIZE,
+				      SLAB_PANIC, NULL);
 }
-core_initcall(pgd_cache_init);