zil_parse: make callback parameters const

Code cleanup, a follow up commit to 4d55ea81.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Co-authored-by: Ryan Moeller <ryan@freqlabs.com>
Signed-off-by: Christian Schwarz <me@cschwarz.com>
Closes #11020