RDMA/netdev: Fix netlink support in IPoIB

IPoIB netlink support was broken by the below commit since integrating
the rdma_netdev support relies on an allocation flow for netdevs that
was controlled by the ipoib driver while netdev's rtnl_newlink
implementation assumes that the netdev will be allocated by netlink.
Such situation leads to crash in __ipoib_device_add, once trying to
reuse netlink device.

This patch fixes the kernel oops for both mlx4 and mlx5
devices triggered by the following command:

Fixes: cd565b4b ("IB/IPoIB: Support acceleration options callbacks")
Signed-off-by: Denis Drozdov <denisd@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Feras Daoud <ferasda@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>