systemd: set restart=always for zfs-zed.service

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Melikov <mail@gmelikov.ru>
Co-authored-by: Attila Fülöp <attila@fueloep.org>
Closes #14294