net: vmxnet3: check for device_active before write

Vmxnet3 device emulator does not check if the device is active,
before using it for write. It leads to a use after free issue,
if the vmxnet3_io_bar0_write routine is called after the device is
deactivated. Add check to avoid it.

Reported-by: Li Qiang <liqiang6-s@360.cn>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Acked-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>