workqueue: introduce cpu_singlethread_map

The code like

	if (is_single_threaded(wq))
		do_something(singlethread_cpu);
	else {
		for_each_cpu_mask(cpu, cpu_populated_map)
			do_something(cpu);
	}

looks very annoying. We can add "static cpumask_t cpu_singlethread_map" and
simplify the code. Lessens .text a bit, and imho makes the code more readable.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>