rst-malloc: Do not grow sh-remap memory

The mremap() grow on anonymous shared memory doesn't produce
usable segment, as the underlying kernel tmpfs file doesn't
get truncated. So any access to new mem will result in sigbus.

Disable this grow for now, we only need few of such memory.

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>