IB/mlx5: Add flow steering support

Adding flow steering support by creating a flow-table per
priority (if rules exist in the priority). mlx5_ib uses
autogrouping and thus only creates the required destinations.

Also includes adding of these flow steering utilities

1. Parsing verbs flow attributes hardware steering specs.

2. Check if flow is multicast - this is required in order to decide
to which flow table will we add the steering rule.

3. Set outer headers in flow match criteria to zeros.

Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>