btrfs: merge the two calls to btrfs_add_ordered_extent in run_delalloc_nocow

Refactor run_delalloc_nocow a little bit so that there is only a single
call to btrfs_add_ordered_extent instead of two.

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>