Staging: hv: Move the definition of struct storvsc_request_extension

In preperation for embedding struct storvsc_request_extension into
struct hv_storvsc_request, move the definition of
struct storvsc_request_extension.

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>