kernel: net: pppol2tp: level handling in pppol2tp_[s,g]etsockopt()

A flaw was found in the way the pppol2tpsetsockopt and pppol2tpgetsockopt functions in the Linux kernel's PPP over L2TP implementation handled requests with a non-SOLPPPOL2TP socket option level. A local, unprivileged user could use this flaw to escalate their privileges on the system...