selftests: forwarding: devlink_lib: Avoid double sourcing of lib.sh

Don't source lib.sh twice and make the script work with ifnames passed
on the command line.

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>