btrfs: fix iterator with update error in backref.c

Fix the following error:

fs/btrfs/backref.c:565:1-20: iterator with update on line 577

Fixes: a7ca4225

('btrfs: use list_for_each_entry* in backref.c')
Signed-off-by: Geliang Tang <geliangtang@163.com>
Signed-off-by: David Sterba <dsterba@suse.com>