selftests/bpf/xdp_redirect_multi: Use arping to accurate the arp number

The arp request number triggered by ping none exist address is not accurate,
which may lead the test false negative/positive. Change to use arping to
accurate the arp number. Also do not use grep pattern match for dot.

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/20211027033553.962413-3-liuhangbin@gmail.com