devlink: add explicitly locked flavor of the rate node APIs

We'll need an explicitly locked rate node API for netdevsim
to switch eswitch mode setting to locked.

Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>