selftests: mptcp: add helpers to get subflow_info

selftests: mptcp: add helpers to get subflow_info

This patch adds 'get_subflow_info' in 'mptcp_diag', which can check whether
a TCP connection is an MPTCP subflow based on the "INET_ULP_INFO_MPTCP"
with tcp_diag method.

The helper 'print_subflow_info' in 'mptcp_diag' can print the subflow_filed
of an MPTCP subflow for further checking the 'subflow_info' through
inet_diag method.

The example of the whole output should be:

  $ ./mptcp_diag -s "127.0.0.1:10000 127.0.0.1:38984"
  127.0.0.1:10000 -> 127.0.0.1:38984
  It's a mptcp subflow, the subflow info:
   flags:Mec token:0000(id:0)/4278e77e(id:0) seq:9288466187236176036 \
   sfseq:1 ssnoff:2317083055 maplen:215

Co-developed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Gang Yan <yangang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250502-net-next-mptcp-sft-inc-cover-v1-6-68eec95898fb@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>