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 and/or buf itself.

Also, remove the unused kaiocb.backend2 field.

Reviewed By: kib
Differential Revision: https://reviews.freebsd.org/D27682