btrfs: introduce btrfs_bio_for_each_block() helper

btrfs: introduce btrfs_bio_for_each_block() helper

Currently if we want to iterate a bio in block unit, we do something
like this:

	while (iter->bi_size) {
		struct bio_vec bv = bio_iter_iovec();

		/* Do something with using the bv */

		bio_advance_iter_single(&bbio->bio, iter, sectorsize);
	}

That's fine for now, but it will not handle future bs > ps, as
bio_iter_iovec() returns a single-page bvec, meaning the bv_len will not
exceed page size.

This means the code using that bv can only handle a block if bs <= ps.

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

- Introduce a helper btrfs_bio_for_each_block()
  Instead of bio_vec, which has single and multiple page version and
  multiple page version has quite some limits, use my favorite way to
  represent a block, phys_addr_t.

  For bs <= ps cases, nothing is changed, except we will do a very
  small overhead to convert phys_addr_t to a folio, then use the proper
  folio helpers to handle the possible highmem cases.

  For bs > ps cases, all blocks will be backed by large folios, meaning
  every folio will cover at least one block. And still use proper folio
  helpers to handle highmem cases.

  With phys_addr_t, we will handle both large folio and highmem
  properly. So there is no better single variable to present a btrfs
  block than phys_addr_t.

- Extract the data block csum calculation into a helper
  The new helper, btrfs_calculate_block_csum() will be utilized by
  btrfs_csum_one_bio().

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

  * index_one_bio() from raid56.c
    Very straight-forward.

  * btrfs_check_read_bio()
    Also update repair_one_sector() to grab the folio using phys_addr_t,
    and do extra checks to make sure the folio covers at least one
    block.
    We do not need to bother bv_len at all now.

  * btrfs_csum_one_bio()
    Now we can move the highmem handling into a dedicated helper,
    calculate_block_csum(), and use btrfs_bio_for_each_block() helper.

There is one exception in btrfs_decompress_buf2page(), which is copying
decompressed data into the original bio, which is not iterating using
block size thus we don't need to bother.

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