IB/core: Add support for IB L2 device-managed steering

This patch adds preliminary support for IB L2 device-managed steering,
currently exposed only in the kernel.

This flow spec can be used by low-level drivers that need to indicate
the link layer type when creating device-managed flow rules.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>