neighbour: make proxy_queue.qlen limit per-device

Right now we have a neigh_param PROXY_QLEN which specifies maximum length
of neigh_table->proxy_queue. But in fact, this limitation doesn't work well
because check condition looks like:
tbl->proxy_queue.qlen > NEIGH_VAR(p, PROXY_QLEN)

The problem is that p (struct neigh_parms) is a per-device thing,
but tbl (struct neigh_table) is a system-wide global thing.

It seems reasonable to make proxy_queue limit per-device based.

v2:
	- nothing changed in this patch
v3:
	- rebase to net tree

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: David Ahern <dsahern@kernel.org>
Cc: Yajun Deng <yajun.deng@linux.dev>
Cc: Roopa Prabhu <roopa@nvidia.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Cc: Alexander Mikhalitsyn <alexander.mikhalitsyn@virtuozzo.com>
Cc: Konstantin Khorenko <khorenko@virtuozzo.com>
Cc: kernel@openvz.org
Cc: devel@openvz.org
Suggested-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Alexander Mikhalitsyn <alexander.mikhalitsyn@virtuozzo.com>
Reviewed-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/neighbour.h

@@ -83,6 +83,7 @@ struct neigh_parms {
 	struct rcu_head rcu_head;
 
 	int	reachable_time;
+	int	qlen;
 	int	data[NEIGH_VAR_DATA_MAX];
 	DECLARE_BITMAP(data_state, NEIGH_VAR_DATA_MAX);
 };