net, neigh: Fix null-ptr-deref in neigh_table_clear()

In the Linux kernel, the following vulnerability has been resolved:

net, neigh: Fix null-ptr-deref in neigh_table_clear()

When IPv6 module gets initialized but hits an error in the middle,

kenel panic with:

KASAN: null-ptr-deref in range [0x0000000000000598-0x000000000000059f]

CPU: 1 PID: 361 Comm: insmod

Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)

RIP: 0010:__neigh_ifdown.isra.0+0x24b/0x370

RSP: 0018:ffff888012677908 EFLAGS: 00000202

...

Call Trace:

<TASK>

neigh_table_clear+0x94/0x2d0

ndisc_cleanup+0x27/0x40 [ipv6]

inet6_init+0x21c/0x2cb [ipv6]

do_one_initcall+0xd3/0x4d0

do_init_module+0x1ae/0x670

...

Kernel panic - not syncing: Fatal exception

When ipv6 initialization fails, it will try to cleanup and calls:

neigh_table_clear()

neigh_ifdown(tbl, NULL)

pneigh_queue_purge(&tbl->proxy_queue, dev_net(dev == NULL))

# dev_net(NULL) triggers null-ptr-deref.

Fix it by passing NULL to pneigh_queue_purge() in neigh_ifdown() if dev

is NULL, to make kernel not panic immediately.