selftests: set conf.all.rp_filter=0 in bareudp.sh

selftests: set conf.all.rp_filter=0 in bareudp.sh

When working on the rp_filter problem, I didn't realise that disabling
it on the network devices didn't cover all cases: rp_filter could also
be enabled globally in the namespace, in which case it would drop
packets, even if the net device has rp_filter=0.

Fixes: 1ccd58331f6f ("selftests: disable rp_filter when testing bareudp")
Fixes: bbbc7aa45eef ("selftests: add test script for bareudp tunnels")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Link: https://lore.kernel.org/r/f2d459346471f163b239aa9d63ce3e2ba9c62895.1605107012.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>