net: sched: cls_u32: simplify the hell out u32_delete() emptiness check

Now that we have the knode count, we can instantly check if
any hnodes are non-empty.  And that kills the check for extra
references to root hnode - those could happen only if there was
a knode to carry such a link.

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>