powerpc/8xx: use pmd_off() to access a PMD entry in pte_update()

The pte_update() implementation for PPC_8xx unfolds page table from the PGD
level to access a PMD entry. Since 8xx has only 2-level page table this can
be simplified with pmd_off() shortcut.

Replace explicit unfolding with pmd_off() and drop defines of pgd_index()
and pgd_offset() that are no longer needed.
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200615092229.23142-1-rppt@kernel.org

arch/powerpc/include/asm/nohash/32/pgtable.h

@@ -205,10 +205,6 @@ static inline void pmd_clear(pmd_t *pmdp)
 	*pmdp = __pmd(0);
 }
 
-/* to find an entry in a page-table-directory */
-#define pgd_index(address)	 ((address) >> PGDIR_SHIFT)
-#define pgd_offset(mm, address)	 ((mm)->pgd + pgd_index(address))
-
 /*
  * PTE updates. This function is called whenever an existing
  * valid PTE is updated. This does -not- include set_pte_at()
@@ -230,6 +226,8 @@ static inline void pmd_clear(pmd_t *pmdp)
  * For other page sizes, we have a single entry in the table.
  */
 #ifdef CONFIG_PPC_8xx
+static pmd_t *pmd_off(struct mm_struct *mm, unsigned long addr);
+
 static inline pte_basic_t pte_update(struct mm_struct *mm, unsigned long addr, pte_t *p,
 				     unsigned long clr, unsigned long set, int huge)
 {
@@ -237,7 +235,7 @@ static inline pte_basic_t pte_update(struct mm_struct *mm, unsigned long addr, p
 	pte_basic_t old = pte_val(*p);
 	pte_basic_t new = (old & ~(pte_basic_t)clr) | set;
 	int num, i;
-	pmd_t *pmd = pmd_offset(pud_offset(p4d_offset(pgd_offset(mm, addr), addr), addr), addr);
+	pmd_t *pmd = pmd_off(mm, addr);
 
 	if (!huge)
 		num = PAGE_SIZE / SZ_4K;