xen-netfront: Fix NULL sring after live migration

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

xen-netfront: Fix NULL sring after live migration

A NAPI is setup for each network sring to poll data to kernel

The sring with source host is destroyed before live migration and

new sring with target host is setup after live migration.

The NAPI for the old sring is not deleted until setup new sring

with target host after migration. With busy_poll/busy_read enabled,

the NAPI can be polled before got deleted when resume VM.

BUG: unable to handle kernel NULL pointer dereference at

0000000000000008

IP: xennet_poll+0xae/0xd20

PGD 0 P4D 0

Oops: 0000 [#1] SMP PTI

Call Trace:

finish_task_switch+0x71/0x230

timerqueue_del+0x1d/0x40

hrtimer_try_to_cancel+0xb5/0x110

xennet_alloc_rx_buffers+0x2a0/0x2a0

napi_busy_loop+0xdb/0x270

sock_poll+0x87/0x90

do_sys_poll+0x26f/0x580

tracing_map_insert+0x1d4/0x2f0

event_hist_trigger+0x14a/0x260

finish_task_switch+0x71/0x230

__schedule+0x256/0x890

recalc_sigpending+0x1b/0x50

xen_sched_clock+0x15/0x20

__rb_reserve_next+0x12d/0x140

ring_buffer_lock_reserve+0x123/0x3d0

event_triggers_call+0x87/0xb0

trace_event_buffer_commit+0x1c4/0x210

xen_clocksource_get_cycles+0x15/0x20

ktime_get_ts64+0x51/0xf0

SyS_ppoll+0x160/0x1a0

SyS_ppoll+0x160/0x1a0

do_syscall_64+0x73/0x130

entry_SYSCALL_64_after_hwframe+0x41/0xa6

...

RIP: xennet_poll+0xae/0xd20 RSP: ffffb4f041933900

CR2: 0000000000000008

---[ end trace f8601785b354351c ]---

xen frontend should remove the NAPIs for the old srings before live

migration as the bond srings are destroyed

There is a tiny window between the srings are set to NULL and

the NAPIs are disabled, It is safe as the NAPI threads are still

frozen at that time