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: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>