mptcp: use mptcp_check_fallback helper

Use __mptcp_check_fallback() helper defined in net/mptcp/protocol.h,
instead of open-coding it in both __mptcp_do_fallback() and
mptcp_diag_fill_info().

Reviewed-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231025-send-net-next-20231025-v1-5-db8f25f798eb@kernel.org


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