btrfs: fold finish_compressed_bio_write into btrfs_finish_compressed_write_work

Fold finish_compressed_bio_write into its only caller as there is no
reason to keep them separate.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>