Drivers: hv: util: Fcopy: Fix a rescind processing issue

Fcopy may use a char device to support the communication between
the user level daemon and the driver. When the Fcopy channel is rescinded
we need to make sure that the char device is fully cleaned up before
we can process a new Fcopy offer from the host. Implement this logic.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>