neigh: hook tracepoints in neigh update code

hook tracepoints at the end of functions that
update a neigh entry. neigh_update gets an additional
tracepoint to trace the update flags and old and new
neigh states.

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