trace: events: neigh_update: print new state in string format

Also, extend neigh_state_str to include neigh dummy states
noarp and permanent

Fixes: 9c03b282 ("trace: events: add a few neigh tracepoints")
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>