xen-netback: free queues after freeing the net device

xen-netback: free queues after freeing the net device

If a queue still has a NAPI instance added to the net device, freeing
the queues early results in a use-after-free.

The shouldn't ever happen because we disconnect and tear down all queues
before freeing the net device, but doing this makes it obviously safe.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 9c6f3ffe8200327d1cf2aad2ff2b414adaacbe96)
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>