ikev2: Fix reauthentication if peer assigns a different virtual IP

ikev2: Fix reauthentication if peer assigns a different virtual IP

Before this change a reqid set on the create_child_t task was used as
indicator of the CHILD_SA being rekeyed.  Only if that was not the case
would the local traffic selector be changed to 0.0.0.0/0|::/0 (as we
don't know which virtual IP the gateway will eventually assign).
On the other hand, in case of a rekeying the VIP is expected to remain
the same, so the local TS would simply equal the VIP.

Since c949a4d5016e33c5 reauthenticated CHILD_SAs also have the reqid
set.  Which meant that the local TS would contain the previously
assigned VIP, basically rendering the gateway unable to assign a
different VIP to the client as the resulting TS would not match
the client's proposal anymore.

Fixes #553.