samples/bpf: Fix buffer overflow in tcp_basertt

Using sizeof(nv) or strlen(nv)+1 is correct.

Fixes: c890063e ("bpf: sample BPF_SOCKET_OPS_BASE_RTT program")
Signed-off-by: Pengcheng Yang <yangpc@wangsu.com>
Link: https://lore.kernel.org/r/1683276658-2860-1-git-send-email-yangpc@wangsu.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

samples/bpf/tcp_basertt_kern.c

@@ -47,7 +47,7 @@ int bpf_basertt(struct bpf_sock_ops *skops)
 		case BPF_SOCK_OPS_BASE_RTT:
 			n = bpf_getsockopt(skops, SOL_TCP, TCP_CONGESTION,
 					   cong, sizeof(cong));
-			if (!n && !__builtin_memcmp(cong, nv, sizeof(nv)+1)) {
+			if (!n && !__builtin_memcmp(cong, nv, sizeof(nv))) {
 				/* Set base_rtt to 80us */
 				rv = 80;
 			} else if (n) {