devlink: Fix netdev notifier chain corruption

Cited commit changed devlink to register its netdev notifier block on
the global netdev notifier chain instead of on the per network namespace
one.

However, when changing the network namespace of the devlink instance,
devlink still tries to unregister its notifier block from the chain of
the old namespace and register it on the chain of the new namespace.
This results in corruption of the notifier chains, as the same notifier
block is registered on two different chains: The global one and the per
network namespace one. In turn, this causes other problems such as the
inability to dismantle namespaces due to netdev reference count issues.

Fix by preventing devlink from moving its notifier block between
namespaces.

Reproducer:

 # echo "10 1" > /sys/bus/netdevsim/new_device
 # ip netns add test123
 # devlink dev reload netdevsim/netdevsim10 netns test123
 # ip netns del test123
 [   71.935619] unregister_netdevice: waiting for lo to become free. Usage count = 2
 [   71.938348] leaked reference.

Fixes: 565b4824

 ("devlink: change port event netdev notifier from per-net to global")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20230215073139.1360108-1-idosch@nvidia.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>