ospfd: fix summary-address deletion

ospfd: fix summary-address deletion

When the summary-address is deleted, `ospf_aggr_handle_external_info` is
called for each aggregated route for the cleanup. It needs to find the
corresponding OSPF instance and it does it using the `ei->instance`
which is totally wrong, because it's the instance from which the route
is redistributed, not the local OSPF instance. A pointer to the correct
OSPF instance is already stored in the external_info structure.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>