devlink: Make sure devlink instance and port are in same net namespace

When devlink reload operation is not used, netdev of an Ethernet port may
be present in different net namespace than the net namespace of the
devlink instance.

Ensure that both the devlink instance and devlink port netdev are located
in same net namespace.

Fixes: 070c63f2

 ("net: devlink: allow to change namespaces during reload")
Signed-off-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>