vdso: Don't park vdso/vvar if restoree doesn't have them

Let's just unmap criu's vdso & vvar if restoree doesn't
have them.
This could be fired e.g. by migrating task from vdso-disabled
kernel to vdso-enabled one.

Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>