LXC: don't unmount mounts for shared root

LXC: don't unmount mounts for shared root

Also after commit 5ff9d8a65ce80efb509ce4e8051394e9ed2cd942
vfs: Lock in place mounts from more privileged users,

unprivileged user has no rights to umount the mounts that
inherited from parent mountns.

right now, I have no good idea to fix this problem, we need
to do more research. this patch just skip unmounting these
mounts for shared root.

BTW, I think when libvirt lxc enables user namespace, the
configuation that shares root with host is very rara.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>