m68k: kill page walker compile warning

The recently introduced page walker (walk_page_range()) calls pgd_offset with a
const struct mm_struct pointer, causing the following compile warning on m68k:

    mm/pagewalk.c:111: warning: passing argument 1 of 'pgd_offset' discards qualifiers from pointer target type

Make the `mm' parameter of the inline function pgd_offset() const to shut it
up.
Signed-off-by: NGeert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

include/asm-m68k/motorola_pgtable.h

@@ -191,7 +191,8 @@ static inline pte_t pte_mkcache(pte_t pte)
 #define pgd_index(address)     ((address) >> PGDIR_SHIFT)
 
 /* to find an entry in a page-table-directory */
-static inline pgd_t *pgd_offset(struct mm_struct *mm, unsigned long address)
+static inline pgd_t *pgd_offset(const struct mm_struct *mm,
+				unsigned long address)
 {
 	return mm->pgd + pgd_index(address);
 }