selftests/bpf: Check length of recv in test_sockmap

The value of recv in msg_loop may be negative, like EWOULDBLOCK, so it's
necessary to check if it is positive before accumulating it to bytes_recvd.

Fixes: 16962b24 ("bpf: sockmap, add selftests")
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Jakub Sitnicki <jakub@cloudflare.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/5172563f7c7b2a2e953cef02e89fc34664a7b190.1716446893.git.tanggeliang@kylinos.cn

tools/testing/selftests/bpf/test_sockmap.c

@@ -681,6 +681,7 @@ static int msg_loop(int fd, int iov_count, int iov_length, int cnt,
 				}
 			}
 
+			if (recv > 0)
 				s->bytes_recvd += recv;
 
 			if (opt->check_recved_len && s->bytes_recvd > total_bytes) {