neighbour: no longer hold RTNL in neigh_dump_info()

neigh_dump_table() is already relying on RCU protection.

pneigh_dump_table() is using its own protection (tbl->lock)

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>