[MIPS] Fix USER_PTRS_PER_PGD for 64K page size.

The code in pgtable-64.h assumes TASK_SIZE is always bigger than a first
level PGDIR_SIZE. This is not the case for 64K pages, where task size is
40 bits (1TB) and a pgd entry can map 42 bits. This leads to
USER_PTRS_PER_PGD being zero for 64K pages.
Signed-off-by: NPeter Watkins <treestem@gmail.com>
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

include/asm-mips/pgtable-64.h

@@ -93,8 +93,12 @@
 #define PTRS_PER_PMD	((PAGE_SIZE << PMD_ORDER) / sizeof(pmd_t))
 #define PTRS_PER_PTE	((PAGE_SIZE << PTE_ORDER) / sizeof(pte_t))
 
+#if PGDIR_SIZE >= TASK_SIZE
+#define USER_PTRS_PER_PGD       (1)
+#else
 #define USER_PTRS_PER_PGD	(TASK_SIZE / PGDIR_SIZE)
-#define FIRST_USER_ADDRESS	0
+#endif
+#define FIRST_USER_ADDRESS	0UL
 
 #define VMALLOC_START		MAP_BASE
 #define VMALLOC_END	\