test_tc_tunnel.sh sets up a pair of namespaces connected by a
veth pair to verify encap/decap using bpf_skb_adjust_room.  In
testing this, it uses tunnel links as the peer of the bpf-based
encap/decap.  However because the same IP header is used for inner
and outer IP, when packets arrive at the tunnel interface they will
be dropped by reverse path filtering as those packets are expected
on the veth interface (where the destination IP of the decapped
packet is configured).

To avoid this, ensure reverse path filtering is disabled for the
namespace using tunneling.

Fixes: 98cdabcd ("selftests/bpf: bpf tunnel encap test")
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>