mptcp: sched: reduce size for unused data

Thanks for the previous commit ("mptcp: sched: split get_subflow
interface into two"), the mptcp_sched_data structure is now currently
unused.

This structure has been added to allow future extensions that are not
ready yet. At the end, this structure will not even be used at all when
mptcp_subflow bpf_iter will be supported [1].

Here is a first step to save 64 bytes on the stack for each scheduling
operation. The structure is not removed yet not to break the WIP work on
these extensions, but will be done when [1] will be ready and applied.

Link: https://lore.kernel.org/6645ad6e-8874-44c5-8730-854c30673218@linux.dev

 [1]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250221-net-next-mptcp-pm-misc-cleanup-3-v1-9-2b70ab1cee79@kernel.org


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