CVE-2026-68398 ppp: defer channel free to an RCU grace period to fix pppol2tp RX UAF

In the Linux kernel, the following vulnerability has been resolved: ppp: defer channel free to an RCU grace period to fix pppol2tp RX UAF pppol2tprecv runs in the L2TP UDP-encap softirq RX path: l2tpudpencaprecv - l2tprecvcommon - pppol2tprecv - pppinput&po-chan It runs under rcureadlock holding...