I've extracted a reproducer for another bug.

This one seems to be much closer to what Dmitry reported intially.

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WARNING: CPU: 1 PID: 3892 at net/ipv6/ip6_fib.c:1472 fib6_del+0xa60/0xdc0
Modules linked in:
CPU: 1 PID: 3892 Comm: a.out Not tainted 4.11.0-rc7+ #251
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:16
 dump_stack+0x292/0x398 lib/dump_stack.c:52
 __warn+0x19f/0x1e0 kernel/panic.c:549
 warn_slowpath_null+0x2c/0x40 kernel/panic.c:584
 fib6_del+0xa60/0xdc0 net/ipv6/ip6_fib.c:1472
 fib6_clean_node+0x3ce/0x550 net/ipv6/ip6_fib.c:1652
 fib6_walk_continue+0x577/0x760 net/ipv6/ip6_fib.c:1578
 fib6_walk+0x91/0xf0 net/ipv6/ip6_fib.c:1623
 fib6_clean_tree+0x266/0x3c0 net/ipv6/ip6_fib.c:1695
 __fib6_clean_all+0x1f7/0x3b0 net/ipv6/ip6_fib.c:1711
 fib6_clean_all+0x27/0x30 net/ipv6/ip6_fib.c:1722
 rt6_ifdown+0xf7/0x910 net/ipv6/route.c:2820
 addrconf_ifdown+0x1a3/0x1aa0 net/ipv6/addrconf.c:3541
 addrconf_notify+0x1bb/0x2570 net/ipv6/addrconf.c:3466
 notifier_call_chain+0x145/0x2f0 kernel/notifier.c:93
 __raw_notifier_call_chain kernel/notifier.c:394
 raw_notifier_call_chain+0x2d/0x40 kernel/notifier.c:401
 call_netdevice_notifiers_info+0x51/0x90 net/core/dev.c:1647
 call_netdevice_notifiers net/core/dev.c:1663
 __dev_notify_flags+0x1fd/0x320 net/core/dev.c:6499
 dev_change_flags+0xf5/0x140 net/core/dev.c:6530
 dev_ifsioc+0x62a/0x9f0 net/core/dev_ioctl.c:254
 dev_ioctl+0x249/0x1160 net/core/dev_ioctl.c:532
 sock_do_ioctl+0x94/0xb0 net/socket.c:913
 sock_ioctl+0x28f/0x440 net/socket.c:1004
 vfs_ioctl fs/ioctl.c:45
 do_vfs_ioctl+0x1bf/0x1780 fs/ioctl.c:685
 SYSC_ioctl fs/ioctl.c:700
 SyS_ioctl+0x8f/0xc0 fs/ioctl.c:691
 entry_SYSCALL_64_fastpath+0x1f/0xc2 arch/x86/entry/entry_64.S:204
RIP: 0033:0x7f575ded4347
RSP: 002b:00007fffc94d9948 EFLAGS: 00000217 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007fffc94d9b30 RCX: 00007f575ded4347
RDX: 00007fffc94d9980 RSI: 0000000000008914 RDI: 0000000000000006
RBP: 00000000004004e0 R08: 0000000000000028 R09: 0101010101010101
R10: 0000000000000000 R11: 0000000000000217 R12: 0000000000000000
R13: 00007fffc94d9b30 R14: 0000000000000000 R15: 0000000000000000
---[ end trace 9bda4459ad907043 ]---