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l2tp_tunnel_get walks the tunnel list to find a matching tunnel
instance and if a match is found, its refcount is increased before
returning the tunnel pointer. But when tunnel objects are destroyed,
they are on the tunnel list after their refcount hits zero. Fix this
by moving the code that removes the tunnel from the tunnel list from
the tunnel socket destructor into in the l2tp_tunnel_delete path,
before the tunnel refcount is decremented.

refcount_t: increment on 0; use-after-free.
WARNING: CPU: 3 PID: 13507 at lib/refcount.c:153 refcount_inc+0x47/0x50
Modules linked in:
CPU: 3 PID: 13507 Comm: syzbot_6e6a5ec8 Not tainted 4.16.0-rc2+ #36
Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
RIP: 0010:refcount_inc+0x47/0x50
RSP: 0018:ffff8800136ffb20 EFLAGS: 00010286
RAX: dffffc0000000008 RBX: ffff880017068e68 RCX: ffffffff814d3333
RDX: 0000000000000000 RSI: ffff88001a59f6d8 RDI: ffff88001a59f6d8
RBP: ffff8800136ffb28 R08: 00000000000...