rst-malloc: use rst_mem_alloc_cont to allocate arrays

rst_mem_alloc() returns alligned slices. When we want to
allocate an array element, we don't expect to get a hole between
a new element and an old one.

Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>