net/sched: act_mirred: Add helper inlines to access tcf_mirred info.

The helper function is_tcf_mirred_mirror helps finding whether an action
struct is of type mirred and is configured to be of type mirror.

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