mptcp: fix setsockopt(IP_TOS) subflow locking

The MPTCP implementation of the IP_TOS socket option uses the lockless
variant of the TOS manipulation helper and does not hold such lock at
the helper invocation time.

Add the required locking.

Fixes: ffcacff8 ("mptcp: Support for IP_TOS for MPTCP setsockopt()")
Cc: stable@vger.kernel.org
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/457


Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20231114-upstream-net-20231113-mptcp-misc-fixes-6-7-rc2-v1-4-7b9cd6a7b7f4@kernel.org


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