Fix another dependency loop

zfs-load-key-DATASET.service was gaining an
After=systemd-journald.socket due to its stdout/stderr going to the
journal (which is the default). systemd-journald.socket has an After
(via RequiresMountsFor=/run/systemd/journal) on -.mount. If the root
filesystem is encrypted, -.mount gets an After
zfs-load-key-DATASET.service.

By setting stdout and stderr to null on the key load services, we avoid
this loop.

Reviewed-by: Antonio Russo <antonio.e.russo@gmail.com>
Reviewed-by: InsanePrawn <insane.prawny@gmail.com>
Signed-off-by: Richard Laager <rlaager@wiktel.com>
Closes #10356
Closes #10388