selftests: mptcp: refactor send_query parameters for code clarity

This patch use 'inet_diag_req_v2' instead of 'token' as parameters of
send_query, and construct the req in 'get_mptcpinfo'.

This modification enhances the clarity of the code, and prepare for the
dump_subflow_info.

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-v1-4-68eec95898fb@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>