Revert "xen/blkback: Don't trust the handle from the frontend."

This reverts commit 01c681d4 upstream
(ef56ca64 in this tree), as it wasn't
supposed to have been applied to the stable tree.

Cc: Jan Beulich <jbeulich@suse.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>