selftests: netfilter: ipvs.sh: Explicity disable rp_filter on interface tunl0

Although setup_ns() set net.ipv4.conf.default.rp_filter=0,
loading certain module such as ipip will automatically create a tunl0 interface
in all netns including new created ones. In the script, this is before than
default.rp_filter=0 applied, as a result tunl0.rp_filter remains set to 1
which causes the test report FAIL when ipip module is preloaded.

Before fix:
Testing DR mode...
Testing NAT mode...
Testing Tunnel mode...
ipvs.sh: FAIL

After fix:
Testing DR mode...
Testing NAT mode...
Testing Tunnel mode...
ipvs.sh: PASS

Fixes: 7c8b89ec ("selftests: netfilter: remove rp_filter configuration")
Signed-off-by: Yi Chen <yiche@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>