xen-netback: correctly return errors from netbk_count_requests()

netbk_count_requests() could detect an error, call
netbk_fatal_tx_error() but return 0.  The vif may then be used
afterwards (e.g., in a call to netbk_tx_error().

Since netbk_fatal_tx_error() could set vif->refcnt to 1, the vif may
be freed immediately after the call to netbk_fatal_tx_error() (e.g.,
if the vif is also removed).

Netback thread              Xenwatch thread
-------------------------------------------
netbk_fatal_tx_err()        netback_remove()
                              xenvif_disconnect()
                                ...
                                free_netdev()
netbk_tx_err() Oops!

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Jan Beulich <JBeulich@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reported-by: Christopher S. Aker <caker@theshore.net>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>