xen-netfront: Improve error handling during initialization

xen-netfront: Improve error handling during initialization

This fixes a crash when running out of grant refs when creating many
queues across many netdevs.

* If creating queues fails (i.e. there are no grant refs available),
call xenbus_dev_fatal() to ensure that the xenbus device is set to the
closed state.
* If no queues are created, don't call xennet_disconnect_backend as
netdev->real_num_tx_queues will not have been set correctly.
* If setup_netfront() fails, ensure that all the queues created are
cleaned up, not just those that have been set up.
* If any queues were set up and an error occurs, call
xennet_destroy_queues() to clean up the napi context.
* If any fatal error occurs, unregister and destroy the netdev to avoid
leaving around a half setup network device.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit e2e004acc7cbe3c531e752a270a74e95cde3ea48)

Conflicts:
	drivers/net/xen-netfront.c

We have a additional feature that is probed (feature_staging_gnts), hence
it couldn't directly apply the chunk. But the chunks added/modified are
still the same as the backport.

Orabug: 22817043
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>