selftests/bpf/xdp_redirect_multi: Give tcpdump a chance to terminate cleanly

No need to kill tcpdump with -9.

Fixes: d2329247 ("selftests/bpf: Add xdp_redirect_multi test")
Suggested-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211027033553.962413-4-liuhangbin@gmail.com