sched: Make nr_iowait_cpu() return 32-bit value

Runqueue ->nr_iowait counters are 32-bit anyway.

Propagate 32-bitness into other code, but don't try too hard.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210422200228.1423391-3-adobriyan@gmail.com