Add DD_FIELD string for snapshots_changed property

This commit adds DD_FIELD string used in extensified dsl_dir zap object
for snapshots_changed property.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Umer Saleem <usaleem@ixsystems.com>
Closes #13819