Hello,

syzbot has tested the proposed patch but the reproducer still triggered  
crash:
KASAN: use-after-free Read in l2tp_tunnel_del_work

l2tp_core: tunl 3: fd 0 wrong protocol, got 1, expected 17
==================================================================
BUG: KASAN: use-after-free in l2tp_tunnel_del_work+0x22e/0x240  
net/l2tp/l2tp_core.c:1292
Read of size 8 at addr ffff8801cdbf2520 by task kworker/u4:14/5459

CPU: 1 PID: 5459 Comm: kworker/u4:14 Not tainted 4.15.0+ #35
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Workqueue: l2tp l2tp_tunnel_del_work
Call Trace:
  __dump_stack lib/dump_stack.c:17 [inline]
  dump_stack+0x194/0x257 lib/dump_stack.c:53
  print_address_description+0x73/0x250 mm/kasan/report.c:252
  kasan_report_error mm/kasan/report.c:351 [inline]
  kasan_report+0x25b/0x340 mm/kasan/report.c:409
  __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:430
  l2tp_tunnel_del_work+0x22e/0x240 net/l2tp/l2tp_core.c:1292
  process_one_work+0xbbf/0x1af0 kernel/workqueue.c:2113
  worker_thread+0x223/0x1990 kernel/workqueue.c:2247
  kthread+0x33c/0x400 kernel/kthread.c:238
  ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:542

Allocated by task 13247:
  save_stack+0x43/0xd0 mm/kasan/kasan.c:447
  set_track mm/kasan/kasan.c:459 [inline]
  kasan_kmalloc+0xad/0xe0 mm/kasan/kasan.c:551
  kasan_slab_alloc+0x12/0x20 mm/kasan/kasan.c:489
  kmem_cache_alloc+0x12e/0x760 mm/slab.c:3541
  sock_alloc_inode+0x70/0x300 net/socket.c:244
  alloc_inode+0x65/0x180 fs/inode.c:209
  new_inode_pseudo+0x69/0x190 fs/inode.c:891
  sock_alloc+0x41/0x270 net/socket.c:565
  __sock_create+0x148/0x850 net/socket.c:1249
  sock_create net/socket.c:1325 [inline]
  SYSC_socket net/socket.c:1355 [inline]
  SyS_socket+0xeb/0x1d0 net/socket.c:1335
  entry_SYSCALL_64_fastpath+0x29/0xa0

Freed by task 13264:
  save_stack+0x43/0xd0 mm/kasan/kasan.c:447
  set_track mm/kasan/kasan.c:459 [inline]
  kasan_slab_free+0x71/0xc0 mm/kasan/kasan.c:524
  __cache_free mm/slab.c:3485 [inline]
  kmem_cache_free+0x83/0x2a0 mm/slab.c:3743
  sock_destroy_inode+0x56/0x70 net/socket.c:274
  destroy_inode+0x15d/0x200 fs/inode.c:266
  evict+0x57e/0x920 fs/inode.c:571
  iput_final fs/inode.c:1516 [inline]
  iput+0x7b9/0xaf0 fs/inode.c:1543
  dentry_unlink_inode+0x4b0/0x5e0 fs/dcache.c:371
  __dentry_kill+0x3de/0x700 fs/dcache.c:575
  dentry_kill fs/dcache.c:616 [inline]
  dput.part.21+0x6fb/0x830 fs/dcache.c:826
  dput+0x1f/0x30 fs/dcache.c:790
  __fput+0x51c/0x7e0 fs/file_table.c:227
  ____fput+0x15/0x20 fs/file_table.c:243
  task_work_run+0x199/0x270 kernel/task_work.c:113
  tracehook_notify_resume include/linux/tracehook.h:191 [inline]
  exit_to_usermode_loop+0x275/0x2f0 arch/x86/entry/common.c:165
  prepare_exit_to_usermode arch/x86/entry/common.c:195 [inline]
  syscall_return_slowpath+0x490/0x550 arch/x86/entry/common.c:264
  entry_SYSCALL_64_fastpath+0x9e/0xa0

The buggy address belongs to the object at ffff8801cdbf2500
  which belongs to the cache sock_inode_cache of size 992
The buggy address is located 32 bytes inside of
  992-byte region [ffff8801cdbf2500, ffff8801cdbf28e0)
The buggy address belongs to the page:
page:ffffea000736fc80 count:1 mapcount:0 mapping:ffff8801cdbf2080  
index:0xffff8801cdbf2ffd
flags: 0x2fffc0000000100(slab)
raw: 02fffc0000000100 ffff8801cdbf2080 ffff8801cdbf2ffd 0000000100000003
raw: ffffea0006eb0420 ffffea000736fee0 ffff8801d9fea380 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
  ffff8801cdbf2400: fb fb fb fb fb fb fb fb fb fb fb fb fc fc fc fc
  ffff8801cdbf2480: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ffff8801cdbf2500: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                ^
  ffff8801cdbf2580: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
  ffff8801cdbf2600: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================


Tested on  
https://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.git/master  
commit
617aebe6a97efa539cc4b8a52adccd89596e6be0 (Sun Feb 4 00:25:42 2018 +0000)
Merge tag 'usercopy-v4.16-rc1' of  
git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux

compiler: gcc (GCC) 7.1.1 20170620
Patch is attached.
Kernel config is attached.
Raw console output is attached.