xfs: kill struct xfs_dir2_sf

The list field of it is never cactually used, so all uses can simply be
replaced with the xfs_dir2_sf_hdr_t type that it has as first member.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>