net: tc_skbedit: Make the skbedit priority offloadable

The skbedit action "priority" is used for adjusting SKB priority. Allow
drivers to offload the action by introducing two new skbedit getters and a
new flow action, and initializing appropriately in tc_setup_flow_action().

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