aio: if there is at least one aio thread, hide an error from aio_init_aioinfo()
Noted and reviewed by: jhb
Fixes: 4685fa8e4bef169e6a1ceaf07f149232326de805
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D51191