btrfs: scrub: introduce dedicated helper to scrub simple-stripe based range

The new entrance will iterate through each data stripe which belongs to
the target device.

And since inside each data stripe, RAID0 is just SINGLE, while RAID10 is
just RAID1, we can reuse scrub_simple_mirror() to do the scrub properly.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>