devlink: Hold rtnl lock while reading netdev attributes

A netdevice of a devlink port can be moved to different net namespace
than its parent devlink instance.
This scenario occurs when devlink reload is not used.

When netdevice is undergoing migration to net namespace, its ifindex
and name may change.

In such use case, devlink port query may read stale netdev attributes.

Fix it by reading them under rtnl lock.

Fixes: bfcd3a46 ("Introduce devlink infrastructure")
Signed-off-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>