I was unable to reproduce this on 4.13 until Nov 23 when it happened again.

This time the logs show that I was handling PDF files via Nautilus which doesn't imply as much disk I/O as the previous 40G dd did.

A quote from:
https://patchwork.kernel.org/patch/9614361/
{{ I am able to reproduce the problem by adding articial [artificial] delays in
__blkdev_put() and writeback_sb_inodes(). Please see the repro patch
below: }}

Nov 23 06:20:38 lakshmi kernel: BUG: unable to handle kernel NULL pointer dereference at           (null)
Nov 23 06:20:38 lakshmi kernel: IP: locked_inode_to_wb_and_lock_list+0x26/0x110
Nov 23 06:20:38 lakshmi kernel: PGD 0 
Nov 23 06:20:39 lakshmi kernel: P4D 0 
Nov 23 06:20:39 lakshmi kernel: 
Nov 23 06:20:39 lakshmi kernel: Oops: 0000 [#1] SMP
Nov 23 06:20:39 lakshmi kernel: Modules linked in: ufs qnx4 hfsplus hfs minix ntfs msdos jfs vhost_net vhost tap uas usb_storage xt_C
Nov 23 06:20:39 lakshmi kernel:  ppdev lp parport ip_tables x_tables autofs4 xfs btrfs raid10 raid456 async_raid6_recov async_memcpy 
Nov 23 06:20:39 lakshmi kernel: CPU: 2 PID: 67 Comm: kswapd0 Not tainted 4.13.0-16-generic #19-Ubuntu
Nov 23 06:20:39 lakshmi kernel: Hardware name: System manufacturer System Product Name/M4A89GTD-PRO/USB3, BIOS 3029    07/05/2012
Nov 23 06:20:39 lakshmi kernel: task: ffff99631ae28000 task.stack: ffffb98741d98000
Nov 23 06:20:39 lakshmi kernel: RIP: 0010:locked_inode_to_wb_and_lock_list+0x26/0x110
Nov 23 06:20:39 lakshmi kernel: RSP: 0018:ffffb98741d9bbc8 EFLAGS: 00010292
Nov 23 06:20:39 lakshmi kernel: RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff9961358ddc68
Nov 23 06:20:39 lakshmi kernel: RDX: 0000000000000001 RSI: 0000000005080020 RDI: ffff9961358dea58
Nov 23 06:20:39 lakshmi kernel: RBP: ffffb98741d9bbf0 R08: 0000000000000000 R09: ffff99629957efc8
Nov 23 06:20:39 lakshmi kernel: R10: 0000000000000228 R11: ffff99629957cd90 R12: ffff9961358dea58
Nov 23 06:20:39 lakshmi kernel: R13: ffff9961358deae0 R14: 0000000000000000 R15: 00000000000000e3
Nov 23 06:20:39 lakshmi kernel: FS:  0000000000000000(0000) GS:ffff996337c80000(0000) knlGS:0000000000000000
Nov 23 06:20:39 lakshmi kernel: CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
Nov 23 06:20:39 lakshmi kernel: CR2: 0000000000000000 CR3: 0000000292009000 CR4: 00000000000006e0
Nov 23 06:20:39 lakshmi kernel: Call Trace:
Nov 23 06:20:39 lakshmi kernel:  inode_io_list_del+0x23/0x50
Nov 23 06:20:39 lakshmi kernel:  evict+0x55/0x1a0
Nov 23 06:20:39 lakshmi kernel:  dispose_list+0x39/0x50
Nov 23 06:20:39 lakshmi kernel:  prune_icache_sb+0x5a/0x80
Nov 23 06:20:39 lakshmi kernel:  super_cache_scan+0x134/0x1b0
Nov 23 06:20:39 lakshmi kernel:  shrink_slab.part.48+0x1d6/0x3d0
Nov 23 06:20:39 lakshmi kernel:  shrink_slab+0x1b/0x30
Nov 23 06:20:39 lakshmi kernel:  shrink_node+0x11e/0x300
Nov 23 06:20:39 lakshmi kernel:  kswapd+0x2cc/0x750
Nov 23 06:20:39 lakshmi kernel:  kthread+0x125/0x140
Nov 23 06:20:39 lakshmi kernel:  ? mem_cgroup_shrink_node+0x180/0x180
Nov 23 06:20:39 lakshmi kernel:  ? kthread_create_on_node+0x70/0x70
Nov 23 06:20:39 lakshmi kernel:  ret_from_fork+0x25/0x30
Nov 23 06:20:39 lakshmi kernel: Code: 00 00 00 00 00 0f 1f 44 00 00 55 48 89 e5 41 56 41 55 41 54 53 4c 8d af 88 00 00 00 49 89 fc 48
Nov 23 06:20:39 lakshmi kernel: RIP: locked_inode_to_wb_and_lock_list+0x26/0x110 RSP: ffffb98741d9bbc8
Nov 23 06:20:39 lakshmi kernel: CR2: 0000000000000000
Nov 23 06:20:39 lakshmi kernel: ---[ end trace c0c5ab8e419a23c0 ]---

I'm going to put on 4.14.2 and see
http://kernel.ubuntu.com/~kernel-ppa/mainline/v4.14.2/