net/sched: fix ->get helper of the matchall cls

It returned always NULL, thus it was never possible to get the filter.

Example:
$ ip link add foo type dummy
$ ip link add bar type dummy
$ tc qdisc add dev foo clsact
$ tc filter add dev foo protocol all pref 1 ingress handle 1234 \
	matchall action mirred ingress mirror dev bar

Before the patch:
$ tc filter get dev foo protocol all pref 1 ingress handle 1234 matchall
Error: Specified filter handle not found.
We have an error talking to the kernel

After:
$ tc filter get dev foo protocol all pref 1 ingress handle 1234 matchall
filter ingress protocol all pref 1 matchall chain 0 handle 0x4d2
  not_in_hw
        action order 1: mirred (Ingress Mirror to device bar) pipe
        index 1 ref 1 bind 1

CC: Yotam Gigi <yotamg@mellanox.com>
CC: Jiri Pirko <jiri@mellanox.com>
Fixes: fd62d9f5c5 ("net/sched: matchall: Fix configuration race")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_matchall.c

@@ -130,6 +130,11 @@ static void mall_destroy(struct tcf_proto *tp, bool rtnl_held,
 
 static void *mall_get(struct tcf_proto *tp, u32 handle)
 {
+	struct cls_mall_head *head = rtnl_dereference(tp->root);
+
+	if (head && head->handle == handle)
+		return head;
+
 	return NULL;
 }