L2ARC: Stop rebuild before setting spa_final_txg

Without doing that there is a race window on export when history
log write by completed rebuild dirties transaction beyond final,
triggering assertion.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Amanakis <gamanakis@gmail.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16714
Closes #16782