aoe: use bio->bi_idx

Instead of starting with bio->bi_io_vec, use the offset in bio->bi_idx.

Signed-off-by: "Ed L. Cashin" <ecashin@coraid.com>
Acked-by: Alan Cox <alan@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>