Initialize ZIL buffers
When populating a ZIL destination buffer ensure it is always
zeroed before its contents are constructed.
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Tom Caputi <caputit1@tcnj.edu>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11687