neighbour: Fix null-ptr-deref in neigh_flush_dev().

neighbour: Fix null-ptr-deref in neigh_flush_dev().

kernel test robot reported null-ptr-deref in neigh_flush_dev(). [0]

The cited commit introduced per-netdev neighbour list and converted
neigh_flush_dev() to use it instead of the global hash table.

One thing we missed is that neigh_table_clear() calls neigh_ifdown()
with NULL dev.

Let's restore the hash table iteration.

Note that IPv6 module is no longer unloadable, so neigh_table_clear()
is called only when IPv6 fails to initialise, which is unlikely to
happen.

[0]:
IPv6: Attempt to unregister permanent protocol 136
IPv6: Attempt to unregister permanent protocol 17
Oops: general protection fault, probably for non-canonical address 0xdffffc00000001a0: 0000 [#1] SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000d00-0x0000000000000d07]
CPU: 1 UID: 0 PID: 1 Comm: systemd Tainted: G                T  6.12.0-rc6-01246-gf7f52738637f #1
Tainted: [T]=RANDSTRUCT
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
RIP: 0010:neigh_flush_dev.llvm.6395807810224103582+0x52/0x570
Code: c1 e8 03 42 8a 04 38 84 c0 0f 85 15 05 00 00 31 c0 41 83 3e 0a 0f 94 c0 48 8d 1c c3 48 81 c3 f8 0c 00 00 48 89 d8 48 c1 e8 03 <42> 80 3c 38 00 74 08 48 89 df e8 f7 49 93 fe 4c 8b 3b 4d 85 ff 0f
RSP: 0000:ffff88810026f408 EFLAGS: 00010206
RAX: 00000000000001a0 RBX: 0000000000000d00 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffffc0631640
RBP: ffff88810026f470 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: ffffffffc0625250 R14: ffffffffc0631640 R15: dffffc0000000000
FS:  00007f575cb83940(0000) GS:ffff8883aee00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f575db40008 CR3: 00000002bf936000 CR4: 00000000000406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 __neigh_ifdown.llvm.6395807810224103582+0x44/0x390
 neigh_table_clear+0xb1/0x268
 ndisc_cleanup+0x21/0x38 [ipv6]
 init_module+0x2f5/0x468 [ipv6]
 do_one_initcall+0x1ba/0x628
 do_init_module+0x21a/0x530
 load_module+0x2550/0x2ea0
 __se_sys_finit_module+0x3d2/0x620
 __x64_sys_finit_module+0x76/0x88
 x64_sys_call+0x7ff/0xde8
 do_syscall_64+0xfb/0x1e8
 entry_SYSCALL_64_after_hwframe+0x67/0x6f
RIP: 0033:0x7f575d6f2719
Code: 08 89 e8 5b 5d c3 66 2e 0f 1f 84 00 00 00 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d b7 06 0d 00 f7 d8 64 89 01 48
RSP: 002b:00007fff82a2a268 EFLAGS: 00000246 ORIG_RAX: 0000000000000139
RAX: ffffffffffffffda RBX: 0000557827b45310 RCX: 00007f575d6f2719
RDX: 0000000000000000 RSI: 00007f575d584efd RDI: 0000000000000004
RBP: 00007f575d584efd R08: 0000000000000000 R09: 0000557827b47b00
R10: 0000000000000004 R11: 0000000000000246 R12: 0000000000020000
R13: 0000000000000000 R14: 0000557827b470e0 R15: 00007f575dbb4270
 </TASK>
Modules linked in: ipv6(+)

Fixes: f7f52738637f4 ("neighbour: Create netdev->neighbour association")
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202507200931.7a89ecd8-lkp@intel.com
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20250723195443.448163-1-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>