virtio: delete napi structures from netdev before releasing memory

free_netdev calls netif_napi_del too, but it's too late, because napi
structures are placed on vi->rq. netif_napi_add() is called from
virtnet_alloc_queues.

general protection fault: 0000 [#1] SMP
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in: ip6table_filter ip6_tables iptable_filter ip_tables virtio_balloon pcspkr virtio_net(-) i2c_pii
CPU: 1 PID: 347 Comm: rmmod Not tainted 3.13.0-rc2+ #171
Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
task: ffff8800b779c420 ti: ffff8800379e0000 task.ti: ffff8800379e0000
RIP: 0010:[<ffffffff81322e19>]  [<ffffffff81322e19>] __list_del_entry+0x29/0xd0
RSP: 0018:ffff8800379e1dd0  EFLAGS: 00010a83
RAX: 6b6b6b6b6b6b6b6b RBX: ffff8800379c2fd0 RCX: dead000000200200
RDX: 6b6b6b6b6b6b6b6b RSI: 0000000000000001 RDI: ffff8800379c2fd0
RBP: ffff8800379e1dd0 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000001 R12: ffff8800379c2f90
R13: ffff880037839160 R14: 0000000000000000 R15: 00000000013352f0
FS:  00007f1400e34740(0000) GS:ffff8800bfb00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 00007f464124c763 CR3: 00000000b68cf000 CR4: 00000000000006e0
Stack:
 ffff8800379e1df0 ffffffff8155beab 6b6b6b6b6b6b6b2b ffff8800378391c0
 ffff8800379e1e18 ffffffff8156499b ffff880037839be0 ffff880037839d20
 ffff88003779d3f0 ffff8800379e1e38 ffffffffa003477c ffff88003779d388
Call Trace:
 [<ffffffff8155beab>] netif_napi_del+0x1b/0x80
 [<ffffffff8156499b>] free_netdev+0x8b/0x110
 [<ffffffffa003477c>] virtnet_remove+0x7c/0x90 [virtio_net]
 [<ffffffff813ae323>] virtio_dev_remove+0x23/0x80
 [<ffffffff813f62ef>] __device_release_driver+0x7f/0xf0
 [<ffffffff813f6ca0>] driver_detach+0xc0/0xd0
 [<ffffffff813f5f28>] bus_remove_driver+0x58/0xd0
 [<ffffffff813f72ec>] driver_unregister+0x2c/0x50
 [<ffffffff813ae65e>] unregister_virtio_driver+0xe/0x10
 [<ffffffffa0036942>] virtio_net_driver_exit+0x10/0x6ce [virtio_net]
 [<ffffffff810d7cf2>] SyS_delete_module+0x172/0x220
 [<ffffffff810a732d>] ? trace_hardirqs_on+0xd/0x10
 [<ffffffff810f5d4c>] ? __audit_syscall_entry+0x9c/0xf0
 [<ffffffff81677f69>] system_call_fastpath+0x16/0x1b
Code: 00 00 55 48 8b 17 48 b9 00 01 10 00 00 00 ad de 48 8b 47 08 48 89 e5 48 39 ca 74 29 48 b9 00 02 20 00 00 00
RIP  [<ffffffff81322e19>] __list_del_entry+0x29/0xd0
 RSP <ffff8800379e1dd0>
---[ end trace d5931cd3f87c9763 ]---

Fixes: 986a4f4d (virtio_net: multiqueue support)
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>