xen/netfront: react properly to failing gnttab_end_foreign_access_ref()

When calling gnttab_end_foreign_access_ref() the returned value must
be tested and the reaction to that value should be appropriate.

In case of failure in xennet_get_responses() the reaction should not be
to crash the system, but to disable the network device.

The calls in setup_netfront() can be replaced by calls of
gnttab_end_foreign_access(). While at it avoid double free of ring
pages and grant references via xennet_disconnect_backend() in this case.

This is CVE-2022-23042 / part of XSA-396.

Reported-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
---
V2:
- avoid double free
V3:
- remove pointless initializer (Jan Beulich)