dm-stripe: Enable atomic writes

Set feature flag DM_TARGET_ATOMIC_WRITES.

Similar to md raid0, the chunk size set in stripe_io_hints() limits the
atomic write unit max.

Signed-off-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>