btrfs: open code end_extent_writepage in end_bio_extent_writepage

This prepares for switching to more efficient ordered_extent processing
and already removes the forth and back conversion from len to end back to
len.

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