Currently, iflink of the parent interface was always accessed, even
when interface didn't have a parent and hence we crashed there.

Handle the interface types properly: for a child interface, return
the ifindex of the parent, for parent interface, return its ifindex.

For child devices, make sure to set the parent pointer prior to
invoking register_netdevice(), this allows the new ndo to be called
by the stack immediately after the child device is registered.

Fixes: 5aa7add8 ('infiniband/ipoib: implement ndo_get_iflink')
Reported-by: NHonggang Li <honli@redhat.com>
Signed-off-by: NErez Shitrit <erezsh@mellanox.com>
Signed-off-by: NHonggang Li <honli@redhat.com>
Reviewed-By: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>+
Acked-by: NNicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>