selftests: disable rp_filter when testing bareudp

Some systems have rp_filter=1 as default configuration. This breaks
bareudp.sh as the intermediate namespaces handle part of the routing
with regular IPv4 routes but the reverse path is done with tc
(flower/tunnel_key/mirred).

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Link: https://lore.kernel.org/r/28140b7d20161e4f766b558018fe2718f9bc1117.1604767577.git.gnault@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>