selftests: forwarding: devlink_lib: Simplify deduction of DEVLINK_DEV

Use devlink -j and jq for more accurate querying. Use cut -f-2 instead
of rev-cut-rev combo.

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