We don't delete napi from hash list during module exit. This will
cause the following panic when doing module load and unload:

BUG: unable to handle kernel paging request at 0000004e00000075
IP: [<ffffffff816bd01b>] napi_hash_add+0x6b/0xf0
PGD 3c5d5067 PUD 0
Oops: 0000 [#1] SMP
...
Call Trace:
[<ffffffffa0a5bfb7>] init_vqs+0x107/0x490 [virtio_net]
[<ffffffffa0a5c9f2>] virtnet_probe+0x562/0x791815639d880be [virtio_net]
[<ffffffff8139e667>] virtio_dev_probe+0x137/0x200
[<ffffffff814c7f2a>] driver_probe_device+0x7a/0x250
[<ffffffff814c81d3>] __driver_attach+0x93/0xa0
[<ffffffff814c8140>] ? __device_attach+0x40/0x40
[<ffffffff814c6053>] bus_for_each_dev+0x63/0xa0
[<ffffffff814c7a79>] driver_attach+0x19/0x20
[<ffffffff814c76f0>] bus_add_driver+0x170/0x220
[<ffffffffa0a60000>] ? 0xffffffffa0a60000
[<ffffffff814c894f>] driver_register+0x5f/0xf0
[<ffffffff8139e41b>] register_virtio_driver+0x1b/0x30
[<ffffffffa0a60010>] virtio_net_driver_init+0x10/0x12 [virtio_net]

This patch fixes this by doing this in virtnet_free_queues(). And also
don't delete napi in virtnet_freeze() since it will call
virtnet_free_queues() which has already did this.

Fixes 91815639 ("virtio-net: rx busy polling support")
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>