net: sched: fix typo in walker_check_empty()

Function walker_check_empty() incorrectly verifies that tp pointer is not
NULL, instead of actual filter pointer. Fix conditional to check the right
pointer. Adjust filter pointer naming accordingly to other cls API
functions.

Fixes: 6676d5e4 ("net: sched: set dedicated tcf_walker flag when tp is empty")
Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Reported-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>