net/core/neighbour: fix kmemleak minimal reference count for hash tables

This should be 1 for normal allocations, 0 disables leak reporting.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Reported-by: Cong Wang <xiyou.wangcong@gmail.com>
Fixes: 85704cb8 ("net/core/neighbour: tell kmemleak about hash tables")
Signed-off-by: David S. Miller <davem@davemloft.net>