AIO: remove the kaiocb->bio linkage
Vectored aio will require each aiocb to be associated with multiple bios, so
we can't store a link to the latter from the former. But we don't really
need to. aio_biowakeup already knows the bio it's using, and the other
fields can be stored within the bio itself.
Also, remove the unused kaiocb.backend2 field.