mm: export balance_dirty_pages_ratelimited_flags()

Export the function balance_dirty_pages_ratelimited_flags(). It is now
also called from btrfs.
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Stefan Roesch <shr@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

mm/page-writeback.c

@@ -1933,6 +1933,7 @@ int balance_dirty_pages_ratelimited_flags(struct address_space *mapping,
 	wb_put(wb);
 	return ret;
 }
+EXPORT_SYMBOL_GPL(balance_dirty_pages_ratelimited_flags);
 
 /**
  * balance_dirty_pages_ratelimited - balance dirty memory state.