blockdev: don't set S_DAX for misaligned partitions

The dax code doesn't currently support misaligned partitions,
so disable O_DIRECT via dax until such time as that support
materializes.

Cc: <stable@vger.kernel.org>
Suggested-by: Boaz Harrosh <boaz@plexistor.com>
Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>