mptcp: decrement subflows counter on failed passive join

commit f3ca0ee upstream.

mptcp_pm_allow_new_subflow() increments extra_subflows before
__mptcp_finish_join() on the passive MP_JOIN path.

In case of race conditions, the subflow is dropped without calling
mptcp_close_ssk(), so the counter is not rolled back.

Call mptcp_pm_close_subflow() when the join completion fails to
decrement the subflows counter.

Fixes: 10f6d46 ("mptcp: fix race between MP_JOIN and close")
Cc: stable@vger.kernel.org
Signed-off-by: Chenguang Zhao <zhaochenguang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260722-net-mptcp-misc-fixes-7-2-rc5-v1-1-6fb595bc86ef@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Chenguang Zhao

net/mptcp/protocol.c

@@ -3825,6 +3825,7 @@ bool mptcp_finish_join(struct sock *ssk)
 	mptcp_data_unlock(parent);
 
 	if (!ret) {
+		mptcp_pm_close_subflow(msk);
 err_prohibited:
 		subflow->reset_reason = MPTCP_RST_EPROHIBIT;
 		return false;