ospfd: fix a reference counting issue introduced by commit 4de8bf0011

ospfd: fix a reference counting issue introduced by commit 4de8bf0011

Commit 4de8bf0011 added a return statement to a loop iterating over a
route_table. That loop uses route_top/route_next.

As commit 4de8bf0011 failed to add a route_node_unlock before the
return statement, a reference is leaked when this codepath is taken.

Signed-off-by: Christian Franke <chris@opensourcerouting.org>
Acked-by: Feng Lu <lu.feng@6wind.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>