restore: Bootstrap len is always page-size aligned

The restore_bootstrap_len is restorer, args and rst-mem lenghts.

Restorer is pie_size() from restorer, which is page-size aligned.
Args len is aligned to page-size few lines above.
The remapable rst-mem grows page by page and is aligned too.

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