xfs: free xfs_attrd_log_items correctly

Technically speaking, objects allocated out of a specific slab cache are
supposed to be freed to that slab cache.  The popular slab backends will
take care of this for us, but SLOB famously doesn't.  Fix this, even if
slob + xfs are not that common of a combination.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>