squashfs4: use upstream xz compression header format

squashfs4: use upstream xz compression header format

In the upstream kernel and the upstream squashfs4 tools the xz
compression header looks the following:
struct disk_comp_opts {
        __le32 dictionary_size;
        __le32 flags;
};

We added some other members and also moved some existing members. Place
the members which are already in upstream header at the same position
as in that kernel and add our own at the end. The kernel should not
have a problem when there are some additional members and just ignore
them.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>