Only run pre-mount hook zfs-load-key on systemd

Reviewed-by: Kash Pande <kash@tripleback.net>
Reviewed-by: bunder2015 <omfgbunder@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matthew Thode <mthode@mthode.org>
Closes #7136
Closes #7140