sefltests: netdevsim: wait for devlink instance after netns removal

When devlink instance is put into network namespace and that network
namespace gets deleted, devlink instance is moved back into init_ns.
This is done as a part of cleanup_net() routine. Since cleanup_net()
is called asynchronously from workqueue, there is no guarantee that
the devlink instance move is done after "ip netns del" returns.

So fix this race by making sure that the devlink instance is present
before any other operation.

Reported-by: Amir Tzin <amirtz@nvidia.com>
Fixes: b74c37fd

 ("selftests: netdevsim: add tests for devlink reload with resources")
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Link: https://lore.kernel.org/r/20230220132336.198597-1-jiri@resnulli.us
Signed-off-by: Paolo Abeni <pabeni@redhat.com>