aoe: switch to the new bio_flush_dcache_pages() interface

Cc: "Ed L. Cashin" <ecashin@coraid.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Ilya Loginov <isloginov@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Horton <phorton@bitbox.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>