Avoid waiting in dmu_sync_late_arrival().

The transaction there does not produce any dirty data or log blocks,
so it should not be throttled. All other cases wait for TXG sync, by
which time the log block we are writing will be obsolete, so we can
skip waiting and just return error here instead.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #15096