btrfs: simplify btrfs_backref_release_cache()

We rely on finding all our nodes on the various lists in the backref
cache, when they are all also in the rbtree.  Instead just search
through the rbtree and free everything.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>