net: devlink: store copy netdevice ifindex and ifname to allow port_fill() without RTNL held

To avoid a need to take RTNL mutex in port_fill() function, benefit from
the introduce infrastructure that tracks netdevice notifier events.
Store the ifindex and ifname upon register and change name events.
Remove the rtnl_held bool propagated down to port_fill() function as it
is no longer needed.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>