mm: pagewalk: add back missing variable initializations

mm: pagewalk: add back missing variable initializations

These initializations accidentially got lost during refactoring.

The first one can't actually be used without initialization, because
walk_p4d_range() is only called when one of the 4 callbacks is set, but relying
on this seems fragile.

Link: https://lkml.kernel.org/r/2123960.ggj6I0NvhH@mobilepool36.emlix.com
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>