net: sched: cls_u32: disallow linking to root hnode

Operation makes no sense.  Nothing will actually break if we do so
(depth limit in u32_classify() will prevent infinite loops), but
according to maintainers it's best prohibited outright.

NOTE: doing so guarantees that u32_destroy() will trigger the call
of u32_destroy_hnode(); we might want to make that unconditional.

Test:
tc qdisc add dev eth0 ingress
tc filter add dev eth0 parent ffff: protocol ip prio 100 u32 \
link 800: offset at 0 mask 0f00 shift 6 plus 0 eat match ip protocol 6 ff
should fail with
Error: cls_u32: Not linking to root node
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_u32.c

@@ -797,6 +797,10 @@ static int u32_set_parms(struct net *net, struct tcf_proto *tp,
 				NL_SET_ERR_MSG_MOD(extack, "Link hash table not found");
 				return -EINVAL;
 			}
+			if (ht_down->is_root) {
+				NL_SET_ERR_MSG_MOD(extack, "Not linking to root node");
+				return -EINVAL;
+			}
 			ht_down->refcnt++;
 		}