powerpc/mm: Fix swapper_pg_dir size on 64-bit hash w/64K pages

Recently in commit f6eedbba7a26 ("powerpc/mm/hash: Increase VA range to 128TB"),
we increased H_PGD_INDEX_SIZE to 15 when we're building with 64K pages. This
makes it larger than RADIX_PGD_INDEX_SIZE (13), which means the logic to
calculate MAX_PGD_INDEX_SIZE in book3s/64/pgtable.h is wrong.

The end result is that the PGD (Page Global Directory, ie top level page table)
of the kernel (aka. swapper_pg_dir), is too small.

This generally doesn't lead to a crash, as we don't use the full range in normal
operation. However if we try to dump the kernel pagetables we can trigger a
crash because we walk off the end of the pgd into other memory and eventually
try to dereference something bogus:

  $ cat /sys/kernel/debug/kernel_pagetables
  Unable to handle kernel paging request for data at address 0xe8fece0000000000
  Faulting instruction address: 0xc000000000072314
  cpu 0xc: Vector: 380 (Data SLB Access) at [c0000000daa13890]
      pc: c000000000072314: ptdump_show+0x164/0x430
      lr: c000000000072550: ptdump_show+0x3a0/0x430
     dar: e802cf0000000000
  seq_read+0xf8/0x560
  full_proxy_read+0x84/0xc0
  __vfs_read+0x6c/0x1d0
  vfs_read+0xbc/0x1b0
  SyS_read+0x6c/0x110
  system_call+0x38/0xfc

The root cause is that MAX_PGD_INDEX_SIZE isn't actually computed to be
the max of H_PGD_INDEX_SIZE or RADIX_PGD_INDEX_SIZE. To fix that move
the calculation into asm-offsets.c where we can do it easily using
max().

Fixes: f6eedbba7a26 ("powerpc/mm/hash: Increase VA range to 128TB")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/book3s/64/pgtable.h

@@ -232,10 +232,6 @@ extern unsigned long __pte_frag_nr;
 extern unsigned long __pte_frag_size_shift;
 #define PTE_FRAG_SIZE_SHIFT __pte_frag_size_shift
 #define PTE_FRAG_SIZE (1UL << PTE_FRAG_SIZE_SHIFT)
-/*
- * Pgtable size used by swapper, init in asm code
- */
-#define MAX_PGD_TABLE_SIZE (sizeof(pgd_t) << RADIX_PGD_INDEX_SIZE)
 
 #define PTRS_PER_PTE	(1 << PTE_INDEX_SIZE)
 #define PTRS_PER_PMD	(1 << PMD_INDEX_SIZE)

arch/powerpc/kernel/asm-offsets.c

@@ -400,8 +400,8 @@ int main(void)
 	DEFINE(BUG_ENTRY_SIZE, sizeof(struct bug_entry));
 #endif
 
-#ifdef MAX_PGD_TABLE_SIZE
-	DEFINE(PGD_TABLE_SIZE, MAX_PGD_TABLE_SIZE);
+#ifdef CONFIG_PPC_BOOK3S_64
+	DEFINE(PGD_TABLE_SIZE, (sizeof(pgd_t) << max(RADIX_PGD_INDEX_SIZE, H_PGD_INDEX_SIZE)));
 #else
 	DEFINE(PGD_TABLE_SIZE, PGD_TABLE_SIZE);
 #endif