selftests/bpf: Fix wrong size passed to bpf_setsockopt()

sizeof(new_cc) is not real memory size that new_cc points to; introduce
a new_cc_len to store the size and then pass it to bpf_setsockopt().

Fixes: 31123c03 ("selftests/bpf: bpf_setsockopt tests")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220824013907.380448-1-yangyingliang@huawei.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/progs/setget_sockopt.c

@@ -305,15 +305,19 @@ static int bpf_test_tcp_sockopt(__u32 i, struct loop_ctx *lc)
 	if (t->opt == TCP_CONGESTION) {
 		char old_cc[16], tmp_cc[16];
 		const char *new_cc;
+		int new_cc_len;
 
 		if (bpf_getsockopt(ctx, IPPROTO_TCP, TCP_CONGESTION, old_cc, sizeof(old_cc)))
 			return 1;
-		if (!bpf_strncmp(old_cc, sizeof(old_cc), cubic_cc))
+		if (!bpf_strncmp(old_cc, sizeof(old_cc), cubic_cc)) {
 			new_cc = reno_cc;
-		else
+			new_cc_len = sizeof(reno_cc);
+		} else {
 			new_cc = cubic_cc;
+			new_cc_len = sizeof(cubic_cc);
+		}
 		if (bpf_setsockopt(ctx, IPPROTO_TCP, TCP_CONGESTION, (void *)new_cc,
-				   sizeof(new_cc)))
+				   new_cc_len))
 			return 1;
 		if (bpf_getsockopt(ctx, IPPROTO_TCP, TCP_CONGESTION, tmp_cc, sizeof(tmp_cc)))
 			return 1;