Linux loop devices

containerd 2.2's mount manager runs slower than the shell steps it bundles

containerd 2.2's mount manager took 29.6 to 47.2 ms per Activate call in a dev.to benchmark, against 23.5 to 25.9 ms for truncate, mkfs, losetup and mount. Most of the gap is design overhead, and a panic plus an orphaned loop device from the same runs are the better reason for shim authors to wait.