[MIPS] Fixe the definition of PTRS_PER_PGD

When we use > 4KB's page size the original definition is not consistent
with PGDIR_SIZE. For exeample, if we use 16KB page size the PGDIR_SHIFT is
(14-2) + 14 = 26, PGDIR_SIZE is 2^26，so the PTRS_PER_PGD should be:

	2^32/2^26 = 2^6

but the original definition of PTRS_PER_PGD is 4096 (PGDIR_ORDER = 0).

So, this definition needs to be consistent with the PGDIR_SIZE.

And the new definition is consistent with the PGD init in pagetable_init().
Signed-off-by: NDajie Tan <jiankemeng@gmail.com>
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

include/asm-mips/pgtable-32.h

@@ -57,7 +57,7 @@ extern int add_temporary_entry(unsigned long entrylo0, unsigned long entrylo1,
 #define PMD_ORDER	1
 #define PTE_ORDER	0
 
-#define PTRS_PER_PGD	((PAGE_SIZE << PGD_ORDER) / sizeof(pgd_t))
+#define PTRS_PER_PGD	(USER_PTRS_PER_PGD * 2)
 #define PTRS_PER_PTE	((PAGE_SIZE << PTE_ORDER) / sizeof(pte_t))
 
 #define USER_PTRS_PER_PGD	(0x80000000UL/PGDIR_SIZE)