selftests/bpf: Use recv_timeout() instead of retries

We use non-blocking sockets in those tests, retrying for
EAGAIN is ugly because there is no upper bound for the packet
arrival time, at least in theory. After we fix poll() on
sockmap sockets, now we can switch to select()+recv().

Signed-off-by: Yucong Sun <sunyucong@gmail.com>
Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211008203306.37525-5-xiyou.wangcong@gmail.com