tcp: add bytes not sent to SCM_TIMESTAMPING_OPT_STATS

Add TCP_NLA_BYTES_NOTSENT to SCM_TIMESTAMPING_OPT_STATS that reports
bytes in the write queue but not sent. This is the same metric as
what is exported with tcp_info.tcpi_notsent_bytes.

Signed-off-by: Yousuk Seung <ysseung@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>