ss: mptcp: print out subflows_total counter

ss: mptcp: print out subflows_total counter

A new counter mptcpi_subflows_total has been added in mptcpi_flags
to count the total amount of subflows from mptcp_info including the
initial one into kernel in this commit:

  6ebf6f90ab4a ("mptcp: add mptcpi_subflows_total counter")

This patch prints out this counter into mptcp_stats output.

Acked-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Geliang Tang <geliang.tang@linux.dev>
Signed-off-by: David Ahern <dsahern@kernel.org>