ofproto: Move tun_table and vl_mff_map deletion.

ofproto: Move tun_table and vl_mff_map deletion.

In this patch, we move the tun_table and vl_mff_map deletion in
ofproto_destory__() to be in the following order.
    1. Delete all the flows.
    2. Delete vl_mff_map.
    3. Delete tun_table.
The rationale behind this order is that a flow may use a variable length
mf_field, and a variable length mf_field is defined by a TLV mapping
in tun_table.

Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Joe Stringer <joe@ovn.org>