selftests: netfilter: disable rp_filter on router

Some distros may enable rp_filter by default. After ns1 change addr to
10.0.2.99 and set default router to 10.0.2.1, while the connected router
address is still 10.0.1.1. The router will not reply the arp request
from ns1. Fix it by setting the router's veth0 rp_filter to 0.

Before the fix:
  # ./nft_fib.sh
  PASS: fib expression did not cause unwanted packet drops
  Netns nsrouter-HQkDORO2 fib counter doesn't match expected packet count of 1 for 1.1.1.1
  table inet filter {
          chain prerouting {
                  type filter hook prerouting priority filter; policy accept;
                  ip daddr 1.1.1.1 fib saddr . iif oif missing counter packets 0 bytes 0 drop
                  ip6 daddr 1c3::c01d fib saddr . iif oif missing counter packets 0 bytes 0 drop
          }
  }

After the fix:
  # ./nft_fib.sh
  PASS: fib expression did not cause unwanted packet drops
  PASS: fib expression did drop packets for 1.1.1.1
  PASS: fib expression did drop packets for 1c3::c01d

Fixes: 82944421

 ("selftests: netfilter: add fib test case")
Signed-off-by: Yi Chen <yiche@redhat.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>