Clean up use of zfs_log_create in zfs_dir

zfs_log_create returns void, so there is no reason to cast its return
value to void at the call site.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11994