btrfs: zstd: use folio_iter to handle zstd_decompress_bio()

btrfs: zstd: use folio_iter to handle zstd_decompress_bio()

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

However cb->compressed_folios[] is just a pointer to each folio of the
compressed bio, meaning we can just replace the compressed_folios[]
array by just grabbing the folio inside the compressed bio.

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