linux_ns_itype.t fails due to improper mountpoint cleaning

[CajuM]

When running sudo ./linux_ns_itype.t it produces the following output, then hangs.

ok 1 - inherit namespace cgroup without type
ok 2 - inherit namespace ipc without type
ok 3 - inherit namespace mnt without type
ok 4 - inherit namespace net without type
ok 5 - inherit namespace pid without type
ok 6 - inherit namespace user without type
ok 7 - inherit namespace uts without type

When Runtime.Clean is called it stumbles upon the container's proc mount. Which was not cleaned up since runc relies on the kernel's mechanism to automatically unmount all mountpoints in a namespace when it gets deleted. Since in this configuration the host and guest share the mount namespace, this never happens. And the test hangs.