ZIL: Remove 128K into 2x68K LWB split optimization

To improve 128KB block write performance in case of multiple VDEVs
ZIL used to spit those writes into two 64KB ones. Unfortunately it
was found to cause LWB buffer overflow, trying to write maximum-
sizes 128KB TX_CLONE_RANGE record with 1022 block pointers into
68KB buffer, since unlike TX_WRITE ZIL code can't split it.

This is a minimally-invasive temporary block cloning fix until the
following more invasive prediction code refactoring.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #15634