ZIL: Avoid dbuf_read() in ztest_get_data()

While working on similar patches for zfs and zvol in #15153 I've
forgot about ztest. Update it also so that we test the same code
paths as use in production.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #15301