xfs: don't create a slab cache for filestream items

We only have very few of these around, and allocation isn't that
much of a hot path.  Remove the slab cache to simplify the code,
and to not waste any resources for the usual case of not having
any inodes that use the filestream allocator.

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