Remove single parent assertion from zio_nowait().

We only need to know if ZIO has any parent there. We do not care if
it has more than one, but use of zio_unique_parent() == NULL asserts
that.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #14823