dirty-bitmap: rename bdrv_undo_clear_dirty_bitmap

Use more generic names to reuse the function for bitmap merge in the
following commit.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: John Snow <jsnow@redhat.com>

blockdev.c

@@ -2033,7 +2033,7 @@ static void block_dirty_bitmap_clear_abort(BlkActionState *common)
                                              common, common);
 
     if (state->backup) {
-        bdrv_undo_clear_dirty_bitmap(state->bitmap, state->backup);
+        bdrv_restore_dirty_bitmap(state->bitmap, state->backup);
     }
 }