aio: Allow expressions in NR_IOEVENTS_IN_PAGES macro

The macro is used only in aio_estimate_nr_reqs():
unsigned int k_max_reqs = NR_IOEVENTS_IN_NPAGES(size/PAGE_SIZE);

Which compiler may evaluate as (((PAGE_SIZE*size)/PAGE_SIZE) - ...)
It works as long as PAGE_SIZE is long.
The patches set converts PAGE_SIZE to use sysconf() returning
(unsigned), non-long type and making the aio macro overflowing.

I do not see any value making PAGE_SIZE (unsigned long) typed.

Cc: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>