Staging: hv: storvsc: Rename the context field in hv_storvsc_request

Rename the context field in hv_storvsc_request. As part of this change
fix the type of this field.

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