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

This reverts commit c93c8519 which is
commit 01c681d4 upstream.

It shouldn't have been applied to the 3.4-stable tree, sorry about that.

Reported-by: William Dauchy <wdauchy@gmail.com>
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>