Fix ZIL clone records for legacy holes

Previous code overengineered cloned range calculation by using
BP_GET_LSIZE(). The problem is that legacy holes don't have the
logical size, so result will be wrong. But we also don't need
to look on every block size, since they all must be identical.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16165