hv_netvsc: move start_remove flag to net_device_context

struct netvsc_device is destroyed on mtu change so keeping the
protection flag there is not a good idea. Move it to struct
net_device_context which is preserved.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>