KASAN: use-after-free in l2tp_session_create

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 38f41ec1 Merge 4.4.125 into android-4.4
git tree: android-4.4
console output: https://syzkaller.appspot.com/x/log.txt?x=13839d0b800000
kernel config: https://syzkaller.appspot.com/x/.config?x=d3227609e1874daa
dashboard link: https://syzkaller.appspot.com/bug?extid=ec6c1d8dc2e2c0117b5e
compiler: gcc (GCC) 7.1.1 20170620
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=13bf5ed3800000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+ec6c1d...@syzkaller.appspotmail.com

IPVS: Creating netns size=2552 id=5
IPVS: Creating netns size=2552 id=6
IPVS: Creating netns size=2552 id=7
IPVS: Creating netns size=2552 id=8
==================================================================
BUG: KASAN: use-after-free in l2tp_session_create+0xf94/0x10f0
net/l2tp/l2tp_core.c:1824


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