selftests: fib_rule_tests: enable forwarding before ipv4 from/iif test
As all the testing addresses are in the same subnet and egress device ==
ingress device. We need enable forwarding to get the route entry.
Also disable rp_filer separately as some distributions enable it in
startup scripts.
Fixes: 65b2b4939a64 ("selftests: net: initial fib rule tests") Signed-off-by: Hangbin Liu <liuhangbin@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net>