MIPS: Expand __swp_offset() to carry 40 significant bits for 64-bit kernel.

With CONFIG_MIGRATION, the PFN of the migrating pages is stored in
__swp_offset(), so we must have enough bits to store the largest
possible PFN.  OCTEON NUMA systems have 41 bits of physical address
space, so with 4K pages (12-bits), we need at least 29 bits to store
the PFN.

The current width of 24-bits is too narrow, so expand it all the way
out to 40-bits.  This leaves the low order 16 bits as zero which does
not interfere with any of the PTE bits.
Signed-off-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9315/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/pgtable-64.h

@@ -279,14 +279,14 @@ extern void pgd_init(unsigned long page);
 extern void pmd_init(unsigned long page, unsigned long pagetable);
 
 /*
- * Non-present pages:  high 24 bits are offset, next 8 bits type,
- * low 32 bits zero.
+ * Non-present pages:  high 40 bits are offset, next 8 bits type,
+ * low 16 bits zero.
  */
 static inline pte_t mk_swap_pte(unsigned long type, unsigned long offset)
-{ pte_t pte; pte_val(pte) = (type << 32) | (offset << 40); return pte; }
+{ pte_t pte; pte_val(pte) = (type << 16) | (offset << 24); return pte; }
 
-#define __swp_type(x)		(((x).val >> 32) & 0xff)
-#define __swp_offset(x)		((x).val >> 40)
+#define __swp_type(x)		(((x).val >> 16) & 0xff)
+#define __swp_offset(x)		((x).val >> 24)
 #define __swp_entry(type, offset) ((swp_entry_t) { pte_val(mk_swap_pte((type), (offset))) })
 #define __pte_to_swp_entry(pte) ((swp_entry_t) { pte_val(pte) })
 #define __swp_entry_to_pte(x)	((pte_t) { (x).val })