mm: pagewalk: allow walk_page_range_novma() without mm

mm: pagewalk: allow walk_page_range_novma() without mm

Since e47690d756a7 ("x86: mm: avoid allocating struct mm_struct on the
stack") a pgd can be passed to walk_page_range_novma().  In case it is set
no place in the pagewalk code use the walk.mm anymore, so permit to pass a
NULL mm instead.  It is up to the caller to ensure proper locking on the
pgd in this case.

Link: https://lkml.kernel.org/r/5760214.MhkbZ0Pkbq@devpool047
Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>