hv_netvsc: Process NETDEV_GOING_DOWN on VF hot remove

On VF hot remove, NETDEV_GOING_DOWN is sent to notify the VF is about to
go down. At this time, the VF is still sending/receiving traffic and we
request the VSP to switch datapath.

On completion, the datapath is switched to synthetic and we can proceed
with VF hot remove.

Signed-off-by: Long Li <longli@microsoft.com>
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>