mptcp: sched: split validation part

A new interface .validate has been added in struct bpf_struct_ops
recently. This patch prepares a future struct_ops support by
implementing it as a new helper mptcp_validate_scheduler() for struct
mptcp_sched_ops.

In this helper, check whether the required ops "get_subflow" of struct
mptcp_sched_ops has been implemented.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250413-net-next-mptcp-sched-mib-sft-misc-v2-2-0f83a4350150@kernel.org


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