mptcp: pass right struct to subflow_hmac_valid

subflow_hmac_valid() needs to access the MPTCP socket and the subflow
request, but not the request sock that is passed in argument.

Instead, the subflow request can be directly passed to avoid getting it
via an additional cast.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250413-net-next-mptcp-sched-mib-sft-misc-v2-4-0f83a4350150@kernel.org


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