mptcp: safety check before fallback

mptcp: safety check before fallback

[ Upstream commit db75a16813aabae3b78c06b1b99f5e314c1f55d3 ]

Recently, some fallback have been initiated, while the connection was
not supposed to fallback.

Add a safety check with a warning to detect when an wrong attempt to
fallback is being done. This should help detecting any future issues
quicker.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250224-net-mptcp-misc-fixes-v1-3-f550f636b435@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>