RDMA/device: Add ib_device_set_netdev() as an alternative to get_netdev

The associated netdev should not actually be very dynamic, so for most
drivers there is no reason for a callback like this. Provide an API to
inform the core code about the net dev affiliation and use a core
maintained data structure instead.

This allows the core code to be more aware of the ndev relationship which
will allow some new APIs based around this.

This also uses locking that makes some kind of sense, many drivers had a
confusing RCU lock, or missing locking which isn't right.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>