Staging: hv: netvsc: Get rid of the refcnt field in struct netvsc_device

Get rid of the refcnt field in struct netvsc_device. We implement the following
logic to manage the life cycle of the device: If the device is being destroyed,
we do not allow any outgoing traffic. Furthermore, if the device is being
destroyed, we allow incoming traffic only to drain outgoing traffic. Note that
the driver may send some book keeping messages to the host not known to
upper level Linux code.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>