Staging: hv: storvsc: Get rid of the on_io_completion in hv_storvsc_request

Get rid of the on_io_completion field  in struct hv_storvsc_request. As part of this
relocate the bounce buffer handling code (to avoid having forward declarations).
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>