net: sched: cls_u32: keep track of knodes count in tc_u_common

allows to simplify u32_delete() considerably

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>