trace: events: add a few neigh tracepoints

The goal here is to trace neigh state changes covering all possible
neigh update paths. Plus have a specific trace point in neigh_update
to cover flags sent to neigh_update.

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>