zfs-mount-generator: Fix escaping for /

The correct name for the mount unit for / is "-.mount", not ".mount".

Reviewed-by: InsanePrawn <insane.prawny@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Antonio Russo <antonio.e.russo@gmail.com>
Signed-off-by: Richard Laager <rlaager@wiktel.com>
Closes #9970

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

@@ -156,7 +156,7 @@ ExecStop=@sbindir@/zfs unload-key '${dataset}'"   > "${dest_norm}/${keyloadunit}
   fi
 
   # Escape the mountpoint per systemd policy.
-  mountfile="$(systemd-escape "${p_mountpoint#?}").mount"
+  mountfile="$(systemd-escape --path --suffix=mount "${p_mountpoint}")"
 
   # Parse options
   # see lib/libzfs/libzfs_mount.c:zfs_add_options