ZIL: Change ZIOs issue order.

In zil_lwb_write_issue(), after issuing lwb_root_zio/lwb_write_zio,
we have no right to access lwb->lwb_child_zio. If it was not there,
the first two ZIOs may have already completed and freed the lwb.
ZIOs issue in opposite order from children to parent should keep
the lwb valid till the end, since the lwb can be freed only after
lwb_root_zio completion callback.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #15233