ospfd: fix dangling pointer when exiting from the helper mode

ospfd: fix dangling pointer when exiting from the helper mode

When exiting from the helper mode for a given router after an
unsuccessful graceful restart, removing the neighborship to that
router straight away leads to a dangling pointer in the associated
interface, which inevitably leads to a crash. To solve this
problem, schedule the removal of the neighbor instead of removing
it immediately.

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