Revert "mount: don't create a temporary directory for pivot_root()"

This reverts commit 21d1b2fdb97facf88436d083896311367e4398af.

pivot_root() can move the current root to a non-shared mount. So we are
going to create a temporary private mount in put_old.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
mount: don't mark mounts as private twice

We do the same action twice. It's typo.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
mntns: pivot_root() can move the current root to a non-shared mount

So we need to create a temporary private mount for the old root.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
mount: don't add extra / between a temporary root and mountpoint

Currenlty a generated path contains two slashes successively.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
mount: move code to validate shared mounts in a separate function

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