mptcp: fix possible integer overflow in mptcp_reset_tout_timer

mptcp: fix possible integer overflow in mptcp_reset_tout_timer

[ Upstream commit b169e76ebad22cbd055101ee5aa1a7bed0e66606 ]

In 'mptcp_reset_tout_timer', promote 'probe_timestamp' to unsigned long
to avoid possible integer overflow. Compile tested only.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Signed-off-by: Dmitry Kandybka <d.kandybka@gmail.com>
Link: https://patch.msgid.link/20241107103657.1560536-1-d.kandybka@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>