btrfs: backref: rename and move backref_cache_cleanup()

Since we're releasing all existing nodes/edges, other than cleanup the
mess after error, "release" is a more proper naming here.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>