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>