zfs_fs: Fix errors when not changing mountpoint

Running `zfs set mountpoint=/mnt/my-ds tank/my-ds`, ZFS may try to
unmount the dataset even if the mountpoint didn't change.
To avoid the confusing error message, this command is now only run when
the mountpoint actually changes.

lib/types/zfs_fs.nix

@@ -73,7 +73,7 @@
               "pbkdf2salt"
               "keyformat"
             ];
-            updateOptions = builtins.removeAttrs config.options onetimeProperties;
+            updateOptions = builtins.removeAttrs config.options (onetimeProperties ++ [ "mountpoint" ]);
             mountpoint = config.options.mountpoint or config.mountpoint;
           in
           ''
@@ -84,12 +84,15 @@
             else
               zfs set ${lib.concatStringsSep " " (lib.mapAttrsToList (n: v: "${n}=${v}") updateOptions)} ${config._name}
               ${lib.optionalString (mountpoint != null) ''
-                # zfs will try unmount the dataset to change the mountpoint
-                # but this might fail if the dataset is in use
-                if ! zfs set mountpoint=${mountpoint} ${config._name}; then
-                  echo "Failed to set mountpoint to '${mountpoint}' for ${config._name}." >&2
-                  echo "You may need to run when the pool is not mounted i.e. in a recovery system:" >&2
-                  echo "  zfs set mountpoint=${mountpoint} ${config._name}" >&2
+                if [[ "$(zfs get -H -o value mountpoint ${config._name})" != "${mountpoint}" ]]; then
+                  echo "Changing mountpoint to '${mountpoint}' for ${config._name}..."
+                  # zfs will try unmount the dataset to change the mountpoint
+                  # but this might fail if the dataset is in use
+                  if ! zfs set mountpoint=${mountpoint} ${config._name}; then
+                    echo "Failed to set mountpoint to '${mountpoint}' for ${config._name}." >&2
+                    echo "You may need to run when the pool is not mounted i.e. in a recovery system:" >&2
+                    echo "  zfs set mountpoint=${mountpoint} ${config._name}" >&2
+                  fi
                 fi
               ''}
             ''}