btrfs: lzo: use folio_iter to handle lzo_decompress_bio()

btrfs: lzo: use folio_iter to handle lzo_decompress_bio()

Currently lzo_decompress_bio() is using
compressed_bio->compressed_folios[] array to grab each compressed folio.

This is making the code much easier to read, as we only need to maintain
a single iterator, @cur_in, and can easily grab any random folio using
@cur_in >> min_folio_shift as an index.

However lzo_decompress_bio() itself is ensured to only advance to the
next folio at one time, and compressed_folios[] is just a pointer to
each folio of the compressed bio, thus we have no real random access
requirement for lzo_decompress_bio().

Replace the compressed_folios[] access by a helper, get_current_folio(),
which uses folio_iter and an external folio counter to properly switch
the folio when needed.

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