xen/blkfront: force data bouncing when backend is untrusted

Split the current bounce buffering logic used with persistent grants
into it's own option, and allow enabling it independently of
persistent grants.  This allows to reuse the same code paths to
perform the bounce buffering required to avoid leaking contiguous data
in shared pages not part of the request fragments.

Reporting whether the backend is to be trusted can be done using a
module parameter, or from the xenstore frontend path as set by the
toolstack when adding the device.

This is CVE-2022-33742, part of XSA-403.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>