vmxnet3: Do not fill stats if device is inactive

Guest OS may issue VMXNET3_CMD_GET_STATS even before device was
activated (for example in linux, after insmod but prior net-dev open).

Accessing shared descriptors prior device activation is illegal as the
VMXNET3State structures have not been fully initialized.

As a result, guest memory gets corrupted and may lead to guest OS
crashes.

Fix, by not filling the stats descriptors if device is inactive.

Reported-by: Leonid Shatz <leonid.shatz@ravellosystems.com>
Acked-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Dana Rubin <dana.rubin@ravellosystems.com>
Signed-off-by: Shmulik Ladkani <shmulik.ladkani@ravellosystems.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
