CVE-2025-39673 ppp: fix race conditions in ppp_fill_forward_path

In the Linux kernel, the following vulnerability has been resolved: ppp: fix race conditions in pppfillforwardpath pppfillforwardpath has two race conditions: 1. The ppp-channels list can change between listempty and listfirstentry, as ppplock is not held. If the only channel is deleted in...