[STABLE v4.9 & v4.14] l2tp: Prevent circular locking and use-afer-free issues

Dear Stable,

Please could you apply the following patches to the v4.9 and v4.14 trees:

225eb26489d05 l2tp: don't use inet_shutdown on ppp session destroy d02ba2a6110c5 l2tp: fix race in pppol2tp_release with session object destroy

They fix circular locking dependency and use-after-free issues.

Kind regards, Lee