MPTCP Info counters: start seq numbers ones from 0

[matttbe]

Now that we have these counters from #385 showing the amount of bytes that have been transmitted:

__u64        mptcpi_bytes_retrans;
__u64        mptcpi_bytes_sent;
__u64        mptcpi_bytes_received;
__u64        mptcpi_bytes_acked;

We can probably deprecate the ones showing the seq num:

__u64   mptcpi_write_seq;
__u64   mptcpi_snd_una;
__u64   mptcpi_rcv_nxt;

On older versions, we can probably start them from 0, see idsn and iasn from the first subflow.