selftests: mptcp: add struct params in mptcp_diag

This patch adds a struct named 'params' to save 'target_token' and other
future parameters. This structure facilitates future function expansions.

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-v1-3-68eec95898fb@kernel.org


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