xen/netfront: destroy queues before real_num_tx_queues is zeroed

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

xen/netfront: destroy queues before real_num_tx_queues is zeroed

xennet_destroy_queues() relies on info->netdev->real_num_tx_queues to
delete queues. Since d7dac083414eb5bb99a6d2ed53dc2c1b405224e5
("net-sysfs: update the queue counts in the unregistration path"),
unregister_netdev() indirectly sets real_num_tx_queues to 0. Those two
facts together means, that xennet_destroy_queues() called from
xennet_remove() cannot do its job, because it's called after
unregister_netdev(). This results in kfree-ing queues that are still
linked in napi, which ultimately crashes:

BUG: kernel NULL pointer dereference, address: 0000000000000000
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD 0 P4D 0
Oops: 0000 [#1] PREEMPT SMP PTI
CPU: 1 PID: 52 Comm: xenwatch Tainted: G W 5.16.10-1.32.fc32.qubes.x86_64+ #226
RIP: 0010:free_netdev+0xa3/0x1a0
Code: ff 48 89 df e8 2e e9 00 00 48 8b 43 50 48 8b 08 48 8d b8 a0 fe ff ff 48 8d a9 a0 fe ff ff 49 39 c4 75 26 eb 47 e8 ed c1 66 ff <48> 8b 85 60 01 00 00 48 8d 95 60 01 00 00 48 89 ef 48 2d 60 01 00
RSP: 0000:ffffc90000bcfd00 EFLAGS: 00010286
RAX: 0000000000000000 RBX: ffff88800edad000 RCX: 0000000000000000
RDX: 0000000000000001 RSI: ffffc90000bcfc30 RDI: 00000000ffffffff
RBP: fffffffffffffea0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000001 R12: ffff88800edad050
R13: ffff8880065f8f88 R14: 0000000000000000 R15: ffff8880066c6680
FS: 0000000000000000(0000) GS:ffff8880f3300000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 00000000e998c006 CR4: 00000000003706e0
Call Trace:
<TASK>
xennet_remove+0x13d/0x300 [xen_netfront]
xenbus_dev_remove+0x6d/0xf0
__device_release_driver+0x17a/0x240
device_release_driver+0x24/0x30
bus_remove_device+0xd8/0x140
device_del+0x18b/0x410
? _raw_spin_unlock+0x16/0x30
? klist_iter_exit+0x14/0x20
? xenbus_dev_request_and_reply+0x80/0x80
device_unregister+0x13/0x60
xenbus_dev_changed+0x18e/0x1f0
xenwatch_thread+0xc0/0x1a0
? do_wait_intr_irq+0xa0/0xa0
kthread+0x16b/0x190
? set_kthread_struct+0x40/0x40
ret_from_fork+0x22/0x30
</TASK>

Fix this by calling xennet_destroy_queues() from xennet_uninit(),
when real_num_tx_queues is still available. This ensures that queues are
destroyed when real_num_tx_queues is set to 0, regardless of how
unregister_netdev() was called.

Originally reported at
https://github.com/QubesOS/qubes-issues/issues/7257