pivot_root requires a place where to move an old root. Currently
a temporary directory is created for that, but it doesn't
work if the / directory is read-only.

Actually we can use any existing directory. In this patch,
criu tries to use /tmp and only if it doesn't exist,
criu creates a temporary directory.

https://bugs.openvz.org/browse/OVZ-6778

v2: don't give a constant string to mkdtemp
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>