sched/core: change rq->nr_iowait type to atomic_long_t

sched/core: change rq->nr_iowait type to atomic_long_t

In preparation for storing two separate iowait states in there, bump the
size from a 32-bit to a 64-bit size, for 64-bit kernels. On 32-bit
archs, the type remains a 32-bit size.

Note that on 32-bit, the number of tasks are limited to 0x8000, which
fits just fine in even half of the existing 32-bit atomic_t. For 64-bit,
no such limit exists, hence play it safe and make it a 64-bit atomic.

Signed-off-by: Jens Axboe <axboe@kernel.dk>