s390,dcssblk,dax: Add dax zero_page_range operation to dcssblk driver

Add dax operation zero_page_range for dcssblk driver.

Suggested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
CC: linux-s390@vger.kernel.org
Link: https://lore.kernel.org/r/20200228163456.1587-4-vgoyal@redhat.com


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