selftests/bpf: Fix invalid flag of recv()

SOCK_NONBLOCK flag is only effective during socket creation, not during
recv. Use MSG_DONTWAIT instead.

Signed-off-by: Jiayuan Chen <mrpre@163.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Acked-by: Jakub Sitnicki <jakub@cloudflare.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://patch.msgid.link/20250122100917.49845-5-mrpre@163.com