net: neighbour: fix neigh_dump_info()

Denys found out "ip neigh" output was truncated to
about 54 neighbours.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Denys Fedoryshchenko <denys@visp.net.lb>
Signed-off-by: David S. Miller <davem@davemloft.net>