fs/xfs: Change XFS_MOUNT_DAX to XFS_MOUNT_DAX_ALWAYS

In prep for the new tri-state mount option which then introduces
XFS_MOUNT_DAX_NEVER.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>