powerpc/mm: Don't kmap_atomic() in pte_offset_map() on PPC32

On PPC32, pte_offset_map() does a kmap_atomic() in order to support
page tables allocated in high memory, just like ARM and x86/32.

But since at least 2008 and commit 8054a342 ("powerpc: Remove dead
CONFIG_HIGHPTE"), page tables are never allocated in high memory.

When the page is in low mem, kmap_atomic() just returns the page
address but still disable preemption and pagefault. And it is
not an inlined function, so we suffer function call for no reason.

Make pte_offset_map() the same as pte_offset_kernel() and make
pte_unmap() void, in the same way as PPC64 which doesn't have HIGHMEM.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/03c97f0f6b3790d164822563be80f2fd4713a955.1581932480.git.christophe.leroy@c-s.fr

arch/powerpc/include/asm/book3s/32/pgtable.h

@@ -366,10 +366,8 @@ static inline void __ptep_set_access_flags(struct vm_area_struct *vma,
 	(((address) >> PAGE_SHIFT) & (PTRS_PER_PTE - 1))
 #define pte_offset_kernel(dir, addr)	\
 	((pte_t *) pmd_page_vaddr(*(dir)) + pte_index(addr))
-#define pte_offset_map(dir, addr)		\
-	((pte_t *)(kmap_atomic(pmd_page(*(dir))) + \
-		   (pmd_page_vaddr(*(dir)) & ~PAGE_MASK)) + pte_index(addr))
-#define pte_unmap(pte)		kunmap_atomic(pte)
+#define pte_offset_map(dir, addr)	pte_offset_kernel((dir), (addr))
+static inline void pte_unmap(pte_t *pte) { }
 
 /*
  * Encode and decode a swap entry.

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

@@ -372,10 +372,8 @@ static inline int pte_young(pte_t pte)
 #define pte_offset_kernel(dir, addr)	\
 	(pmd_bad(*(dir)) ? NULL : (pte_t *)pmd_page_vaddr(*(dir)) + \
 				  pte_index(addr))
-#define pte_offset_map(dir, addr)		\
-	((pte_t *)(kmap_atomic(pmd_page(*(dir))) + \
-		   (pmd_page_vaddr(*(dir)) & ~PAGE_MASK)) + pte_index(addr))
-#define pte_unmap(pte)		kunmap_atomic(pte)
+#define pte_offset_map(dir, addr)	pte_offset_kernel((dir), (addr))
+static inline void pte_unmap(pte_t *pte) { }
 
 /*
  * Encode and decode a swap entry.