btrfs: remove buffer_heads from btrfsic_process_written_block()

Now that the last caller of btrfsic_process_written_block() with
buffer_heads is gone, remove the buffer_head processing path from it as
well.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>