selftests/bpf: Fix error return code in run_getsockopt_test()

Fix to return a negative error code from the error handling
case instead of 0, as done elsewhere in this function.

Fixes: 65b4414a ("selftests/bpf: add sockopt test that exercises BPF_F_ALLOW_MULTI")
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NWang Hai <wanghai38@huawei.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20201116101633.64627-1-wanghai38@huawei.com

tools/testing/selftests/bpf/prog_tests/sockopt_multi.c

@@ -138,7 +138,8 @@ static int run_getsockopt_test(struct bpf_object *obj, int cg_parent,
 	 */
 
 	buf = 0x40;
-	if (setsockopt(sock_fd, SOL_IP, IP_TOS, &buf, 1) < 0) {
+	err = setsockopt(sock_fd, SOL_IP, IP_TOS, &buf, 1);
+	if (err < 0) {
 		log_err("Failed to call setsockopt(IP_TOS)");
 		goto detach;
 	}