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 <[e-mail]>
Signed-off-by: Stefan Roesch <[e-mail]>
Reviewed-by: David Sterba <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

mm/page-writeback.c

···
 	wb_put(wb);
 	return ret;
 }
+EXPORT_SYMBOL_GPL(balance_dirty_pages_ratelimited_flags);
 
 /**
  * balance_dirty_pages_ratelimited - balance dirty memory state.