selftests/bpf: fix a CI failure caused by vsock sockmap test

BPF CI has reported the following failure:

Error: #200/79 sockmap_listen/sockmap VSOCK test_vsock_redir
  Error: #200/79 sockmap_listen/sockmap VSOCK test_vsock_redir
  ./test_progs:vsock_unix_redir_connectible:1506: egress: write: Transport endpoint is not connected
  vsock_unix_redir_connectible:FAIL:1506
  ./test_progs:vsock_unix_redir_connectible:1506: ingress: write: Transport endpoint is not connected
  vsock_unix_redir_connectible:FAIL:1506
  ./test_progs:vsock_unix_redir_connectible:1506: egress: write: Transport endpoint is not connected
  vsock_unix_redir_connectible:FAIL:1506
  ./test_progs:vsock_unix_redir_connectible:1514: ingress: recv() err, errno=11
  vsock_unix_redir_connectible:FAIL:1514
  ./test_progs:vsock_unix_redir_connectible:1518: ingress: vsock socket map failed, a != b
  vsock_unix_redir_connectible:FAIL:1518
  ./test_progs:vsock_unix_redir_connectible:1525: ingress: want pass count 1, have 0

It’s because the recv(... MSG_DONTWAIT) syscall in the test case is
called before the queued work sk_psock_backlog() in the kernel finishes
executing. So the data to be read is still queued in psock->ingress_skb
and cannot be read by the user program. Therefore, the non-blocking
recv() reads nothing and reports an EAGAIN error.

So replace recv(... MSG_DONTWAIT) with xrecv_nonblock(), which calls
select() to wait for data to be readable or timeout before calls recv().

Fixes: d61bd8c1

 ("selftests/bpf: add a test case for vsock sockmap")
Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
Link: https://lore.kernel.org/r/20230804073740.194770-4-xukuohai@huaweicloud.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>