btrfs: introduce btrfs_bio_for_each_block_all() helper

Currently if we want to iterate all blocks inside a bio, we do something
like this:

	bio_for_each_segment_all(bvec, bio, iter_all) {
		for (off = 0; off < bvec->bv_len; off += sectorsize) {
			/* Iterate blocks using bv + off */
		}
	}

That's fine for now, but it will not handle future bs > ps, as
bio_for_each_segment_all() is a single-page iterator, it will always
return a bvec that's no larger than a page.

But for bs > ps cases, we need a full folio (which covers at least one
block) so that we can work on the block.

To address this problem and handle future bs > ps cases better:

- Introduce a helper btrfs_bio_for_each_block_all()
  This helper will create a local bvec_iter, which has the size of the
  target bio. Then grab the current physical address of the current
  location, then advance the iterator by block size.

- Use btrfs_bio_for_each_block_all() to replace existing call sites
  Including:

  * set_bio_pages_uptodate() in raid56
  * verify_bio_data_sectors() in raid56

  Both will result much easier to read code.

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

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