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>