Btrfs: dev-replace: make sure target is identical to source when raid56 rebuild fails

In the last step of scrub_handle_error_block, we try to combine good
copies on all possible mirrors, this works fine for raid1 and raid10,
but not for raid56 as it's doing parity rebuild.

If parity rebuild doesn't get back with correct data which matches its
checksum, in case of replace we'd rather write what is stored in the
source device than the data calculuated from parity.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>