Fix use-afer-free regression in RAIDZ expansion

We should not dereference rra after the last zio_nowait() is called.
It seems very unlikely, but ASAN in ztest managed to catch it.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16868