selftests/bpf: Set rp_filter in test_flow_dissector

Many distributions enable rp_filter. However, the flow dissector test
generates packets that have 1.1.1.1 set as (inner) source address without
this address being reachable. This causes the selftest to fail.

The selftests should not assume a particular initial configuration. Switch
off rp_filter.

Fixes: 50b3ed57

 ("selftests/bpf: test bpf flow dissection")
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Petar Penkov <ppenkov@google.com>
Link: https://lore.kernel.org/bpf/513a298f53e99561d2f70b2e60e2858ea6cda754.1570539863.git.jbenc@redhat.com