RDMA/nldev: Return device protocol

Add new RDMA_NLDEV_ATTR_DEV_PROTOCOL attribute to give ability for UDEV
rules create IB device stable names based on link type protocol.  The
assumption that devices like mlx4 with duality in their link type under
one IB device struct won't be allowed in the future.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>