criu: rename current_ns_mask to root_ns_mask (v2)

Now we supports sub-mntns, so root_ns_mask sounds more correct than
current_ns_mask.

v2: typo fix
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>