libnvdimm: initialize struct blk_integrity with 0

Initialize struct blk_integrity with 0 as blk_integrity_register() takes the
then unitialized struct blk_integrity::flags and ORs it to the resulting block
integrity structure.

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>