ospfd: fix another DR election issue during graceful restart

ospfd: fix another DR election issue during graceful restart

Commit 3551ee9e90304 introduced a regression that causes GR to fail
under certain circumstances. In short, while ISM events should
be ignored while acting as a helper for a restarting router, the
DR/BDR fields of the neighbor structure should still be updated
while processing a Hello packet. If that isn't done, it can cause
the helper to elect the wrong DR while exiting from the helper mode,
leading to a situation where there are two DRs for the same network
segment (and a failed GR by consequence). Fix this.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>