tcp: define initial scaling factor value as a macro

So that other users could access it. Notably MPTCP will use
it in the next patch.

No functional change intended.

Acked-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231023-send-net-next-20231023-2-v1-4-9dc60939d371@kernel.org


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