network: neighbor: Always add neighbors with replace

We were duplicating setting flags for the message and a combination of
NLM_F_APPEND and NLM_F_CREATE which does not make sense. We should have
been using NLM_F_REPLACE and NLM_F_CREATE since the kernel can
dynamically create neighbors prior to us adding an entry. Otherwise, we
can end up with cases where the message will time out after ~25s even
though the neighbor still gets added. This delays the rest of the setup
of the interface even though the error is ultimately ignored.