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 openzfs#14823

module/zfs/zio.c

@@ -2341,7 +2341,7 @@ zio_nowait(zio_t *zio)
 	ASSERT3P(zio->io_executor, ==, NULL);
 
 	if (zio->io_child_type == ZIO_CHILD_LOGICAL &&
-	    zio_unique_parent(zio) == NULL) {
+	    list_is_empty(&zio->io_parent_list)) {
 		zio_t *pio;
 
 		/*