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.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>