selftests: devlink_lib: Fix bouncing of netdevsim DEVLINK_DEV

In the commit referenced below, a check was added to devlink_lib that
asserts the existence of a devlink device referenced by $DEVLINK_DEV.
Unfortunately, several netdevsim tests point DEVLINK_DEV at a device that
does not exist at the time that devlink_lib is sourced. Thus these tests
spuriously fail.

Fix this by introducing an override. By setting DEVLINK_DEV to an empty
string, the user declares their intention to handle DEVLINK_DEV management
on their own.

In all netdevsim tests that use devlink_lib and set DEVLINK_DEV, set
instead an empty DEVLINK_DEV just before sourcing devlink_lib, and set it
to the correct value right afterwards.

Fixes: 557c4d2f780c ("selftests: devlink_lib: add check for devlink device existence")
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

selftests: netdevsim: Add test for resilient nexthop groups offload API

Test various aspects of the resilient nexthop group offload API on top
of the netdevsim implementation. Both good and bad flows are tested.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Co-developed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

selftests: netdevsim: Add test for nexthop offload API

Test various aspects of the nexthop offload API on top of the netdevsim
implementation. Both good and bad flows are tested.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>