Originally it's iterating all the sectors which has dbitmap sector for
the vertical stripe.

It can be easily converted to sector bytenr iteration with an test_bit()
call.
Signed-off-by: NQu Wenruo <wqu@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>