dm,dax: Add dax zero_page_range operation

This patch adds support for dax zero_page_range operation to dm targets.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Acked-by: Mike Snitzer <snitzer@redhat.com>
Link: https://lore.kernel.org/r/20200228163456.1587-5-vgoyal@redhat.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>