IB/IPoIB: Introduce RDMA netdev interface and IPoIB structs

Add RDMA netdev interface to ib device structure allowing RDMA
netdev devices to be allocated by ib clients.

The idea is to allow to providers to optimize IPoIB data path.
New struct that includes functions and data member is exposed.
It exposes set of callback functions for handling data path flows
in IPoIB driver.

Each provider can support these set of functions in order
to optimize its specific data path, and let IPoIB to leverage
its data path.

There is an assumption, that providers should give the full set
of functions and not only part of them, in order to work properly.

Signed-off-by: Erez Shitrit <erezsh@mellanox.com>
Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>