zil: Relax assertion in zil_parse

Rather than panic debug builds when we fail to parse a whole ZIL, let's
instead improve the logging of errors and continue like in a release
build.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #14116