[NEIGH]: neigh_table_clear() doesn't free stats

neigh_table_clear() doesn't free tbl->stats.
Found by Alexey Kuznetsov. Though Alexey considers this
leak minor for mainstream, I still believe that cleanup
code should not forget to free some of the resources :)

At least, this is critical for OpenVZ with virtualized
neighbour tables.

Signed-Off-By: Kirill Korotaev <dev@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>