Comment # 2 on bug 1099312 from Jeff Mahoney

I've just backported:

commit 38e3eebff643db725633657d1d87a3be019d1018
Author: Josef Bacik <josef@toxicpanda.com>
Date:   Wed Jan 16 11:00:57 2019 -0500

    btrfs: honor path->skip_locking in backref code

to the stable branch.