devlink: Simplify devlink_pernet_pre_exit call

The devlink_pernet_pre_exit() will be called if net namespace exits.

That routine is relevant for devlink instances that were assigned to
that namespaces first. This assignment is possible only with the following
command: "devlink reload DEV netns ...", which already checks reload support.

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>