ofproto-dpif-xlate: Fix segmentation fault caused by tun_table

ofproto-dpif-xlate: Fix segmentation fault caused by tun_table

Currently, the revalidator thread may hit segmentation fault when geneve
TLV map is updated.  It is because we may store the old TLV map (struct
tun_table) in the frozen state for recirculation, and we may access the
already freed old tun_table in xlate_actions().

This patch update the logic of getting tun_table so that we will use
the latest tun_table instead of the frozen one.

VMWare-BZ: #2106987
Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>