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: 65b2b493 ("selftests: net: initial fib rule tests")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>