btrfs: zstd: introduce zstd_compress_bio() helper

btrfs: zstd: introduce zstd_compress_bio() helper

The new helper has the following enhancements against the existing
zstd_compress_folios()

- Much smaller parameter list

  No more shared IN/OUT members, no need to pre-allocate a
  compressed_folios[] array.

  Just a workspace and compressed_bio pointer, everything we need can be
  extracted from that @cb pointer.

- Ready-to-be-submitted compressed bio

  Although the caller still needs to do some common works like
  rounding up and zeroing the tailing part of the last fs block.

Overall the workflow is the same as zstd_compress_folios(), but with
some minor changes:

- @start/@len is now constant
  For the current input file offset, use @start + @tot_in instead.

  The original change of @start and @len makes it pretty hard to know
  what value we're really comparing to.

- No more @cur_len
  It's only utilized when switching input buffer.
  Directly use btrfs_calc_input_length() instead.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>