btrfs: scrub: use scrub_simple_mirror() to handle RAID56 data stripe scrub

Although RAID56 has complex repair mechanism, which involves reading the
whole full stripe, but inside one data stripe, it's in fact no different
than SINGLE/RAID1.

The point here is, for data stripe we just check the csum for each
extent we hit.  Only for csum mismatch case, our repair paths divide.

So we can still reuse scrub_simple_mirror() for RAID56 data stripes,
which saves quite some code.

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