nr_req always depends on ring size and we even check for that.

Kill parasite_aio::max_reqs,vma_nr_reqs and vma_area::aio_nr_req,
because they are not need and excess.
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>