mount: don't destruct an external mount-namespace (v2)

If a parent mount point is shared with exteranl mntns, a child will be
umounted from the external mntns too.

For example:
$ mount -t tmpfs xxx /root/tmp/
$ mount --make-shared tmp
$ mkdir tmp/xxx
$ mount -t tmpfs xxx /root/tmp/xxx
$ touch tmp/xxx/a

$ unshare -m umount tmp/xxx
$ ls -l tmp/xxx/a
ls: cannot access tmp/xxx/a: No such file or directory

This patch changes a parent mnt to private for umounting childrens.

v2: exit if a mount point can not be marked ad private

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>