zpool: fix default dataset getting shadowed

If the zpool's root dataset is not the rootfs and gets mounted on
creation, the actual rootfs will get mounted later and shadow the
current mountpoint.

Running `zfs unmount` is the easiest way to unmount the zpool's root
dataset on creation without messing up the value of the `mountpoint`
setting.

lib/types/zpool.nix

@@ -67,6 +67,9 @@
           ${lib.concatStringsSep " " (lib.mapAttrsToList (n: v: "-o ${n}=${v}") config.options)} \
           ${lib.concatStringsSep " " (lib.mapAttrsToList (n: v: "-O ${n}=${v}") config.rootFsOptions)} \
           "''${zfs_devices[@]}"
+        ${lib.optionalString ((config.rootFsOptions.mountpoint or "") != "none") ''
+          zfs unmount ${config.name}
+        ''}
         ${lib.concatMapStrings (dataset: dataset._create) (lib.attrValues config.datasets)}
       '';
     };