fs/xfs: Update xfs_ioctl_setattr_dax_invalidate()

Because of the separation of FS_XFLAG_DAX from S_DAX and the delayed
setting of S_DAX, data invalidation no longer needs to happen when
FS_XFLAG_DAX is changed.

Change xfs_ioctl_setattr_dax_invalidate() to be
xfs_ioctl_dax_check_set_cache() and alter the code to reflect the new
functionality.

Furthermore, we no longer need the locking so we remove the join_flags
logic.

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