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

etc/systemd/system-generators/zfs-mount-generator.in

@@ -221,6 +221,10 @@ ${keymountdep}
 [Service]
 Type=oneshot
 RemainAfterExit=yes
+# This avoids a dependency loop involving systemd-journald.socket if this
+# dataset is a parent of the root filesystem.
+StandardOutput=null
+StandardError=null
 ExecStart=${keyloadcmd}
 ExecStop=${keyunloadcmd}"   > "${dest_norm}/${keyloadunit}"
     fi