Staging: hv: Introduce a function to map a generic driver pointer to a pointer...

Staging: hv: Introduce a function to map a generic driver pointer to a pointer to storvsc_driver_object

In preparation for getting rid of the priv element from struct hv_driver,
introduce a function that maps a generic struct driver pointer to struct
storvsc_driver_object.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Abhishek Kane <v-abkane@microsoft.com>
Signed-off-by: Hank Janssen <hjanssen@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>