This happens because we unconditionally attempt to initialize device
stats on mount, but we may not have been able to read the device root.
Fix this by skipping initializing the device stats if we do not have a
device root.
Reported-by: Neal Gompa <ngompa13@gmail.com> CC: stable@vger.kernel.org # 5.11+ Signed-off-by: Josef Bacik <josef@toxicpanda.com> Reviewed-by: David Sterba <dsterba@suse.com> Signed-off-by: David Sterba <dsterba@suse.com>