xen-blkback: don't leak stack data via response ring

Rather than constructing a local structure instance on the stack, fill
the fields directly on the shared ring, just like other backends do.
Build on the fact that all response structure flavors are actually
identical (the old code did make this assumption too).

This is XSA-216.

Cc: stable@vger.kernel.org

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
