xfs: don't forbid setting dax flag on directories if device doesn't dax

On a directory, the DAX flag is merely a hint that files created in the
directory should have the DAX flag set at creation time.  We don't care
if the underlying device supports DAX or not because directory metadata
are always cached in DRAM.  We don't care if new files get the flag even
if the device doesn't support DAX because we always check for DAX
support before setting the VFS flag (S_DAX).

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