net/sched: properly assign RCU pointer in tcf_chain_tp_insert/remove

*p_filter_chain is rcu-dereferenced on reader path. So here in writer,
property assign the pointer.

Fixes: 2190d1d0 ("net: sched: introduce helpers to work with filter chains")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>