*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: NJiri Pirko <jiri@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>