rst alloc: align on reporting cpos too

Since we align in rst_mem_alloc, we should also align when reporting the
current position; if we don't and things get unlucky, we report a different
position than where the pointer is actually allocated, which fucks things
up quite bad :)

Closes #111

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>