selftests: forwarding: lib: Parameterize NUM_NETIFS in two functions

setup_wait() and tc_offload_check() both assume that all NUM_NETIFS
interfaces are relevant for a given test. However, the scale test script
acts as an umbrella for a number of sub-tests, some of which may not
require all the interfaces.

Thus it's suboptimal for tc_offload_check() to query all the interfaces.
In case of setup_wait() it's incorrect, because the sub-test in question
of course doesn't configure any interfaces beyond what it needs, and
setup_wait() then ends up waiting indefinitely for the extraneous
interfaces to come up.

For that reason, give setup_wait() and tc_offload_check() an optional
parameter with a number of interfaces to probe. Fall back to global
NUM_NETIFS if the parameter is not given.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Yuval Mintz <yuvalm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>