RDMA/netdev: Use priv_destructor for netdev cleanup

Now that the unregister_netdev flow for IPoIB no longer relies on external
code we can now introduce the use of priv_destructor and
needs_free_netdev.

The rdma_netdev flow is switched to use the netdev common priv_destructor
instead of the special free_rdma_netdev and the IPOIB ULP adjusted:
 - priv_destructor needs to switch to point to the ULP's destructor
   which will then call the rdma_ndev's in the right order
 - We need to be careful around the error unwind of register_netdev
   as it sometimes calls priv_destructor on failure
 - ULPs need to use ndo_init/uninit to ensure proper ordering
   of failures around register_netdev

Switching to priv_destructor is a necessary pre-requisite to using
the rtnl new_link mechanism.

The VNIC user for rdma_netdev should also be revised, but that is left for
another patch.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Denis Drozdov <denisd@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>