net: sched: teach tcf_bind/unbind_filter to use block->q

Whenever the block->q is set, it can be used instead of tp->q as it
contains the same value. When it is not set, which can't happen now but
it might happen with the follow-up shared blocks introduction, the class
is not set in the result. That would lead to a class lookup instead
of direct class pointer use for classful qdiscs. However, it is not
planned to support classful qdisqs sharing filter blocks, so that may
never happen.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>