net: sched: matchall: verify that filter is not NULL in mall_walk()

Check that filter is not NULL before passing it to tcf_walker->fn()
callback. This can happen when mall_change() failed to offload filter to
hardware.

Fixes: ed76f5edcc ("net: sched: protect filter_chain list with filter_chain_lock mutex")
Reported-by: Ido Schimmel <idosch@mellanox.com>
Tested-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_matchall.c

@@ -247,6 +247,9 @@ static void mall_walk(struct tcf_proto *tp, struct tcf_walker *arg,
 
 	if (arg->count < arg->skip)
 		goto skip;
+
+	if (!head)
+		return;
 	if (arg->fn(tp, head, arg) < 0)
 		arg->stop = 1;
 skip: