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

I found this solution in the LXC code. We can open the old root, call
pivot_root(".", "."), call fchdir to the old root and call umount(".").

Now restore will not fail, if the root is read-only.
In addition it's a bit faster.

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