selftests: forwarding: lib: Support NUM_NETIFS of 0

So far the case of NUM_NETIFS of 0 has not been interesting. However if
one wishes to reuse the lib.sh routines in a setup of a separate
namespace, being able to import like this is handy.

Therefore replace the {1..$NUM_NETIFS} references, which cause iteration
over 1 and 0, with an explicit for loop like we do in setup_wait() and
tc_offload_check(), so that for NUM_NETIFS of 0 no iteration is done.

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>