xfs: remove xfs_filestream_associate

There is no good reason to create a filestream when a directory entry
is created.  Delay it until the first allocation happens to simply
the code and reduce the amount of mru cache lookups we do.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>