net: sched: cls_flower: use flow_dissector for ingress ifindex

Use previously introduced infra to obtain and store ingress ifindex
instead doing it locally.

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