This is possible if spa_sync() completes before the ZIL write/flush
does, which then errors. At this point all itxs are in the past, leaving
us with nothing to wait for.
In a perfect world we would not fail the ZIL, but at this point we've
already locked out the itxgs early, so we have to see it through.
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>