cpumask: limit FORCE_NR_CPUS to just the UP case

Hardcoding the number of CPUs at compile time does improve code
generation, but if you get it wrong the result will be confusion.

We already limited this earlier to only "experts" (see commit
fe5759d5 "cpumask: limit visibility of FORCE_NR_CPUS"), but with
distro kernel configs often having EXPERT enabled, that turns out to not
be much of a limit.

To quote the philosophers at Disney: "Everyone can be an expert. And
when everyone's an expert, no one will be".

There's a runtime warning if you then set nr_cpus to anything but the
forced number, but apparently that can be ignored too [1] and by then
it's pretty much too late anyway.

If we had some real way to limit this to "embedded only", maybe it would
be worth it, but let's see if anybody even notices that the option is
gone.  We need to simplify kernel configuration anyway.

Link: https://lore.kernel.org/all/20240618105036.208a8860@rorschach.local.home/

 [1]
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Paul McKenney <paulmck@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>