xfs: fix the dax supported check in xfs_ioctl_setattr_dax_invalidate

Setting the DAX flag on the directory of a file system that is not on a
DAX capable device makes as little sense as setting it on a regular file
on the same file system.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>