xen-netback: correctly return errors from netbk_count_requests()
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 <a5240e9e4c4d742e3a4232ca749f78defd45d722@citrix.com>
Signed-off-by: Jan Beulich <051346888327db48cce19d2cfd0e1c700bb7cf16@suse.com>
Signed-off-by: David Vrabel <b9d45bd1f671e508cd8daa63dfc3cabb597562df@citrix.com>
Reported-by: Christopher S. Aker <5ebeb37e634851d8c41b1c400da40a9508c039e7@theshore.net>
Acked-by: Ian Campbell <9be92b3dbbd6611e3e4c9209fb3d04b4919e7cca@citrix.com>
Signed-off-by: David S. Miller <fe08d3c717adf2ae63592e4c9aec6e3e404d8e3e@davemloft.net>

KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas